DMBSI 



UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 835 

Contribution from the Bureau of Public Roads 
THOMAS H. MACDONALD, Chief 



Washington, D. C. 



PROFESSIONAL PAPER 



August 6, 1920 



CAPILLARY MOVEMENT OF 
SOIL MOISTURE 

By 
WALTER W. McLaughlin, Senior Irrigation Engineer 



CONTENTS 



Page 

Object 2 

Plan of Experiments 2 

Rate and Extent of Movement of Soil 

Moisture by Capillarity 13 

Effect of Gravity on the Movement of 

Soil Moisture by Capillarity .... 39 

Evaluation of Empirical Curves . . . . 47 



Open Versus Covered Flumes .... 54 
Effect of Temperature on Soil-Moisture 

Conditions 56 

The Capillary Siphon 68 

Capillary Movement of Moisture From a 

Wet to a Dry Soil 63 

References 69 




WASHINGTON 
GOVERNMENT PRINTING OFFICE 

1920 



L^tT' 




i7 6f, 1^9 






UNITED STATES DEPARTMENT OF AGRICULTURE 




^ BULLETIN No. 835 



Contribution from the Bureau of Public Roads 
THOMAS H. MacDONALD, Chief 




jn-f^^<»=f«. 



Washington, D. C. 



PROFESSIONAL PAPER 



August 6, 1920 



CAPILLARY MOVEMENT OF SOIL MOISTURE, 

By Walter W. McLaughlin, Finiior Irrlnntinn Engineer. 



CONTENTS. 



Pago. 

Object 2 

Plan of experiments 2 

Pate and <>xtont of movement of soil 

moisture by capillarity 13 

Effect of gravity on tbe movement of 

soil moisture bj- capillarity 39 

Evaluation of empirical curves 47 



Page. 

Open versus covered flumes 54 

Effect of temperature on soil-moisture 

conditions 56 

The capillary siphon 58 

Capillary movement of moisture from 

a wet to a di-y soil 63 

References 69 



The irrigation engineer has long felt the need of more detailed 
information as to the importance of capillarity as a source of loss 
of water from irrigation works and the part it plays in distributing, 
within the soil, water applied in irrigation. It has long been recog- 
nized that impounding reservoirs and conveying channels lose more 
water than can be accounted for by direct percolation and evapora- 
tion. Whether this loss was the result of capillary action alone or 
in combination with the transpiration from plant growth along canal 
banks has been only a matter of conjecture. Wliere the water ap- 
plied to soil by irrigation goes and how it ultimately distributes itself 
within the soil have been questions of speculation. 

It has been observed that the percentage of moisture determined in 
the field in the usual way has not always given a true basis upon 
which to determine the necessity of applying water by irrigation. 
In some instances, the percentages of moisture determined have been 
above the wilting point and yet plants were wilted. 

This condition has caused the irrigation engineer to speculate 
upon the probability of the rate of movement of soil moisture from 
one point to another by capillarity, as well as the extent to which the 
moisture may move. 

The irrigator is always confronted by questions of methods of 
irrigation, duration of irrigation, and frequency of irrigation. The 

147697°— Bull. 835—20 — — 1 



2 BULLETIN 835, IT. S. DEPARTMENT OF AGEICULTURE. 

first aim is to obtain a uniform distribution of moisture within reach 
of the phmt roots and to maintain such distribution. The economical 
ai:)plication of water to prevent waste from deep percolation or sur- 
face run-off and to maintain an optimum percentage of moisture 
within the soil is the vital problem. For instance, under specified 
soil and topographic conditions, how long should the furrows be 
and how far apart? With turns of irrigation coming at specified 
intervals, how much water should be applied and how long should 
an irrigation be continued at each turn ? To approximate an accurate^ 
answer to questions of this land it is necessary to know more accu- 
rately than we now know the rate and extent of movement of the 
soil moisture by capillarity during the several periods of an irriga- 
tion season. 

The drainage engineer in the arid region has frequently been per- 
plexed by a condition of water-logging under conditions which seem 
to preclude the possibility of the movement of free water as sucli 
from any known source to the wet area. He has often felt a want of 
specific information which would indicate the development of free 
water from capillary moisture and the importance of this form of 
moisture in farm drainage. 

OBJECT. 

As a basis for answering some of the above questions investiga- 
tions were undertaken in 191.5, and the data given below are in the 
form of a progTess report. 

The object of these experiments is to furnish specific data as to the 
capillary movement of moisture in the soils of the arid region. It 
is felt that these will be of prime importance to the irrigation en- 
gineer in the proper construction and operation of conveying chan- 
nels and impounding reservoirs, and that they will enable him to 
point out the most economical methods of applying water to fields. 
These data were obtained for different soils and under different con- 
ditions. 

PLAN OF EXPERIMENTS. 

Because it was realized that the rate and extent of movement of 
moisture in soils by capillarity differs materially where the source 
of moisture is a body of free water from where it is a body of wet 
soil, the experiments have been divided into two parts : 

1. ^\^iere the source of the moisture is a body of free water into 
which the soil column extends. 

2. lA'liere the source of moisture is a body of soil containing a per- 
centage of moisture greater than the wilting percentage, and not 
connected with a body of free water. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 3 

The work as planned and carried out embodied a study of the rate 
and extent of capilhiry movement of moisture in cohmms of various 
types of soil, where capillarity was assisted by gravity, where it 
acted against gravity, and where gravity as a factor was eliminated. 

The columns in which gravity was to assist capillarity w^ere inclined 
downward at various angles from the horizontal; the columns in 
which gravity was to act against the force of capillar-ity were inclined 
upward at various angles from the horizontal; and the columns in 
which the effect of gravity w^as to be eliminated as far as possible were 
set horizontal. 

Inasmuch as evaporation is one of the factors that controls the 
extent and rate of movement of soil moisture by capillarity, it was 
decided to run each set of experiments in duplicate, except that one 
column wa& to be covered on all sides and evaporation reduced to a 
minimum, while the other column was to be uncovered and exposed 
on one side to the air. 

It was essential to the plan of the experiments that the probability 
of free water as such entering the columns be reduced to a minimum 
and yet have sufficient water enter the flumes to give something with 
which to work. It was desired to have a high initial percentage of 
capillary water, and at the same time eliminate free water. To accom- 
plish this end it was decided to have a vertical lift from the surface 
of the water in the tank to the bottom of the container of the soil 
column proper of from 3 to 4 inches. After several preliminary tests 
a vertical lift of 4 inches was adopted and all columns except the ver- 
tical ones (unless otherwise stated) have a vertical "lift" of 4 inches 
from the surface of the water in the tanks to any chtmge in direction 
of the column. That part of the soil column from the surface of the 
water to the point of change in direction has been termed the " wick "" 
in the discussion which follows. 

Air-tight joints were maintained and no water escaped from the 
tanks except by the wick and no moisture from the columns except 
by evaporation. To guard against the formation of a true siphon 
within the soil column an air space was maintained upon at least one 
side of the soil column throughout its entire length, in the columns 
inclined downward. 

All water added to the tanks after the initial filling was measured 
and recorded. At specified intervals the position of the outward ex- 
tent of the wet area of soil was measured and these measurements' 
recorded. 

The experiments in which a moist soil was the source of moisture 
rather than a body of free water differ but little from those described, 
except that evaporation was eliminated in all cases. 

The soil boxes were partially filled with a soil containing a known 
percentage of moisture, greater than the wilting i^ercentage. and the 



4 BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 

lemtiiiuler of the box filled with air-dry soil packed firmly against the 
wet soil. The boxes were set either vertical or horizontal, no inclined 
boxes being used. In the boxes set vertical, in some experiments, the 
wet soil was placed at the top of the box and the air-dry soil was 
placed at the lower end. In other boxes the wet soil was placed at the 
lower end of the box and the air-dry soil at the upper end. Thus, 
the movement of the moisture from the Avet soil into the dry soil by 
capillarity would be, in some cases, with the force of gravity and, in 
other cases, in an opposite direction. A few vertical boxes had the 
middle section of the box filled with the wet soil, with the air-dry 
soil at both ends, thus combining in the same box and at the same 
time the upward and downward movements. 

The horizontal boxes were packed in the same way as the vertical 
boxes with wet soil at one end and air-dry soil at the other. In a 
few tests the middle section of the horizontal boxes was filled with 
wet soil and air-dry soil placed at both ends. In a very few tests 
the middle part of the box was filled with two sections of wet soil 
containing different initial precentages of moisture and the dry soil 
was placed at both ends of the box. 

METEOROLOGICAL DATA. 

In connection with the experiments a record was kept of the evap- 
oration from a free water surface and a thermograph record taken 
of the air temperature. No other meteorological data were recorded. 

SOILS USED. 

A uniforui surface soil was selected for each set of experiments. 
This soil was to be typical of a large area and was to be of a well- 
known type. The soils were to be obtained from various parts of 
the arid region that the data might be of general value. The greater 
the number of types and the wider the range in types of soils used, 
the greater the value of the tests. Uniform soils were to be used, as 
the movement of moisture by capillarity varies in soils of different 
types and the results obtained with mixed soils would be of little 
value. 

INCIDENTAL EXPERIMENTS. 

. The movement of soil moisture by capillarity within a soil of a 
uniform type differs materiall}^ from its movement between soils 
of different types. This difference is found in the rate and extent 
of movement and in the initial percentage of moisture necessary to 
permit movement. To obtain some light upon this point a few 
experiments were conducted. The general plan of these auxiliary 
experiments was about the same as for the original experiments. In 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 5 

the auxiliary experiments, various types of soil were packed in layers 
or one end of a colunui or box contained soil of one type and the 
other end soil of a different tj^pe. 

METHOD AND EQUIPMENT. 

A confined soil column was used and the method differed from that 
usually employed by other investigators only in the size and arrange- 
ment of soil columns. The columns used in these experiments are 100 
square inches in cross-sectional area and much larger than the col- 
umns usually employed. A feature made important in the present 
work is the use of inclined columns. 

One side and the bottom of each flume were made of wood with 
metal lining and the other side was of plate glass. In the discussion 
of the experiments the term " flume " will be used to designate the 
soil column and its container. 

Uniform soil was packed into the flumes and wicks extended 
from within the water in the tanks up into the flumes. After the 
soil had been i)laced in the flumes the tanks were filled up to the 
initial level and this level rather constantly maintained throughout 
the experiment. 

At 9 a. m. of each day and frequently at other hours the outward 
extent of the soil wetted by capillary moisture was measured, and the 
water in the tanks was brought up to the initial elevation with 
measured quantities of water added directly to the tanks. Soil sam- 
ples were taken at various points in the wet soil area, at such inter- 
vals of time as deemed advisable and always at the end of an ex- 
periment. All the flumes or columns were protected by canvas from 
the direct rays of the sun and fi'om the rain. 

MEASURING THE ADVANCE OF THE CAPILLARY MOISTURE. 

The outward extent of the whetted soil area, indicating the extent 
of the moisture movement at any time, is plainly \dsible through the 
glass side of the flume. The wetted soil is of a darker color and 
the line of demarcation is very distinct. The position of this line as 
seen through the glass side was traced upon the glass. The position 
of these markings with reference to the s.urface of the water in the 
tank is determined by five direct measurements made in the way and 
to the points as follows : 

Five lines are drawn along the glass side of the flume parallel to 
tlie longitudinal axis of the flume. The first line is at the top of 
the glass; the second line is 2^ inches lower; the third is 5 inches 
from the top and at the middle of the glass side; the fourth is 7| 
inches from the top, while the fifth is at the bottom of the flume 
and 10 inches from the top line. The intersections of the marks on 



6 BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 

the side of the flume indicating the outward extent of the wet soil 
area and the fixe lines above described give five definite points with 
wliich to locate each of the markings upon the glass side of the 
flume. The positions of these five points are determined by direct 
measurements from the surface of the water in the tank along the 
five lines parallel to the longitudinal axis of the soil column. 

The original horizontal surface of the water in the tanks was used 
as a base for all measurements of the position of the moisture in the 
soil column in all flumes rather than a transverse line coincident 
with the change in inclination of the soil column, if any, from the ver- 
tical. Inasmuch as the movement of moisture in the soil columns by 
capillarity from free water is about equal for all inclinations, from the 
vertical upward to the vertical downward, for the first foot or more, 
using the surface of the water as a base for measurements does not 
produce an appreciable error in making comparisons. 

In tlie experiments with wet and dry soils the initial point of 
measurement is the line of contact between the original areas of wet 
and dry soil. No water is added to the boxes after they are set up, 
but the water is added to the wet soil at the time of jjacking. The 
quantity of water to be added to the soil to be packed wet is calculated 
upon the dry weight of the soil and then this water is added by 
measurement. 

MAINTAINING THE WATER LEVEL IN TANKS. 

All water added to the tanks after the initial filling is added in 
measured quantities and recorded as water used by the flume. Water 
is added sufficiently often to maintain the level of water in the tanks 
at a rather constant elevation. The water added during any 24 hours 
is recorded as the water used during the day ending at 9 a. m. Unless 
otherv\^ise specified all references to water used per day will mean for 
the day ending at 9 a. m. 

SAMPLING FOR MOISTURE. 

The soil is sampled for moisture with a ;|-inch carpenter's auger in 
the usual waj' and the samples immediately placed in tared screw- 
topped glass bottles and weighed. A composite samj)le is made of 
the upper .5 inches of soil and another composite sample for the lower 
T) inches in each boring. The samples are taken in planes parallel to 
tlie planes indicating the advance of the moisture within the flumes 
at the points sampled. A boring is located by a measurement along 
the top of the flume from the water level. The samples, as soon as 
convenient after the first weighing, are placed in a water- jacketed 
oven and dried at the temperature of boiling water until a constant 
weight is obtained. Using the dry weight of the soil sample as a 
basis, the percentage of moisture in the sample is calculated. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 7 

The samples from the box experiments are taken and treated in the 
same way as for the fiumes, except that one composite sample is made 
for each boring in the boxes. 

PREPARATION OF SOIL FOR PACKING. 

The soil to be used in the experiments is thoroughly air dried, if 
not already so. The soil is spread out in thin layers and exposed to 
the direct rays of the sun for several days. The air-dried soil i.s 
then screened tlu-ough a ^-inch screen and all large rocks, roots, 
etc., removed. Lumps of soil are broken up and screened. The 
heavy clay soils having numerous large lumps are rolled with a hand 
lawn roller and screened. In order that the soil grains may not bo 
broken by the roller, it is necessary to roll upon some rather yielding 
foundation. A soil foundation was made by rolling repeatedly with 
a weighted roller. Soils of the clay and loam type are passed 
through a 14-mesh screen and the screenings from all operations 
thoroughly rriixed. The preparation of the lieavier soils of the 
Whittier type is a slow and tedious operation. It is only by re- 
peated rolling with a light roller that the soils can be properly fined 
without crushing the soil grains. 

SETTING UP THE FLUMES. 

The fiumes were set up out in the open and were protected only 
from the direct rays of the sun and from the rain. They rest upon 
2 by 12 inch plank cut to the proper length and set upon end. The 
tanks rest upon small stands fastened firmly to the foundation for the 
flmnes. Thus the supporting structure for the entire soil column is 
rigid. 

The flume, tank, and ell were set in position, the glass side of the 
flume put in position, and then all joints were filled with melted 
paraffin wax. All joints were tested a second time to see that they 
Avere air and water tight. The flume including the wick was then 
ready for packing. 

PACKING SOIL IN FLUMES. 

The soil was placed in the flume in 2-inch laj'ers and packed with 
a wooden block and hammer. The block is corrugated and is 4 b}' 6 
inches. The packing was done by striking the block with the ham- 
mer, using as uniform a blow as practicable and continuing the pack- 
ing until the soil was of about the same density as found in the 
field. This density was estimated in both instances by measurement 
and weight. The soil was placed and packed into the flumes layer 
by layer until filled. 



8 BULLETiiSr 835, ij. s. departme:nt of agriculture. 

PACKING THE BOXES. 

The boxes were packed with soil in much the same way as the 
flumes, except when the initial percentage of moisture in the wet-soil 
part of the box was relatively low. In this case, the soil was first 
wetted to the desired degree, and then placed in the box in layers one 
inch thick and packed by dropping a weight a given distance upon a 
board covering the layer of soil. The distance the weight was to be 
dropped, and the number of times it was to be dropped for each layer 
was determined by tests for each soil. The section of the box to be 
filled with air-dry soil was packed by using the hammer and block. 

PREVENTING EVAPORATION IN FLUMES. 

Those flumes in which evaporation from tlie top of the flume was 
to be i)i'evented were covered with two-ply unsancled maltoid roof- 
ing paper. A strip of the roofing cut to the proper size was placed 
upon the top of the flume and reached froiu one end to the other. 
The side joints were made air tight. On the glass side of the flume 
the roofing was folded over and down on tlie outside of the glass 
about one-half inch. The joint between the roofing and the glass 
Avas held in place and made tight by means of an angle-iron strip 
made of galvanized iron clamped along the upper edge of the glass 
and on top of the roofing. To prevent air-trapping, ij-inch vent 
holes were cut in the roofing at intervals of about 4 feet. Tests 
of the efi'ectiveness of this covering to prevent evaporation of 
moisture from the flumes indicate that at least 80 per cent of the 
evaporation from an open flume was prevented by this covering. 

A more effective method of preventing evaporation could be de- 
vised, but there woukl be great danger of the entrance of some un- 
known factors into the work. The entrance of these factors would 
prove fatal for comparison with much of the other work. 

COVERING THE BOXES. 

The plate-glass sides of the boxes were sealed to the boxes by means 
of cushions made of maltoid roofing. The glass was held in place and 
clamped tightly to the box by means of wooden strips fastened to 
the box proper by means of eyebolts fitted with thread and nut. 
Rubber cushions were tried, but did not give the same satisfaction 
that was obtained from the use of maltoid. 

CAPILLARY ACTION IN THE SOIL IN THE ABSENCE OF FREE WATER. 

The term " free water " as here used is w^ater not held by capil- 
larity and obeying the laws of gravity. It is variously tei;med " free 
water." " ground water," and " water of gravitation." (17.)^ 

^ The fisurcs in paienthosos apply to tho references at the end of this buIleUn. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 9 

The plan of this experiment was to study the rate and extent of 
movement of moisture from a wetted soil into an air-dry soil when 
the two were brought in contact. The wetted soil was to contain 
various percentages of moisture from near the point of capillary 
saturation down to the wilting point. 

THE SOIL BOXES. 

The soil boxes or soil tubes for this w^ork as first designed con- 
sisted of galvanized iron boxes G by 6 inches in cross section and of 
various lengths from 4 to 8 feet.. 

It was soon found that the metal boxes first used were not suffi- 
ciently rigid. They were difficult to pack and the least jarring of the 
box after it was packed and set in position was very apt to crack the 
soil column. The second set of equipment, the boxes now in use, is 
described later. 

ADDING THE WATER. 

Various methods were tried for adding the water to soil to be 
wetted and at the same time insure a uniform pack offering no 
mechanical obstacle in the movement of the moisture by capillarity. 
The method finally adopted as giving the most uniform results for 
the higher percentages of moisture was found not adapted to the 
smaller percentages of moisture. In the first method, the water was 
added to the soil after it had been packed and its distribution in that 
part of the soil column left to capillary action. In the second method, 
or the one used for the smaller percentages of moisture, the water 
was added before packing. Where the water w'as to be added after 
the soil was packed, a small furrow about 2 inches deep Avas made the 
entire length of the part of the column to be wetted and the proper 
amount of soil would take it up, and finally, wdth the last of the water 
was added that part of the soil removed to make the furrow. The 
wetted soil was then covered with plate glass and allowed to stand 
24 hours before packing the air-dried part of the column. As soon 
as the dry soil was added the plate glass side was placed and sealed 
in position and the box set in place and the experiment was under 
way. 

When the moisture was added before packing, a mass of soil suffi- 
cient for one pack was moistened to the desired percentage by adding 
a weighted quantity of water. The mass was thoroughly mixed by 
turning over and over several times on a piece of oil cloth. This 
soil was then placed in the box in layers 2 inches in thickness and 
tamped with a hard rubber tamping bar. The amount of tamping 
Aviis much a matter of judgment and testing, except that the same 



10 BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 

soil with the same percentage of water used in difl'erent boxes 
received the same amount of tamping. 

MEASURING THE ADVANCE OF MOISTURE. 

The change in color of the soil in the dry part of the colnmn with 
a change in moisture content was very marked in nearly all soils ex- 
cept the light sands, devoid of much organic matter. With the posi- 
tion of the contact of the wet and dry part of the column at the 
commencement of the experiment marked upon the glass side of 
the box, it was a simple matter to measure the distance the moisture 
had mo^ed into the dry part of the column at any time. These 
measurements were recorded, as well as the date and hour of the 
measurement. 

OTHEU OBSERVATIONS OF THE Sf)IE COLL'MN. 

During an experiment and at its expiration close observations were 
made of the condition of the column for cracks or other factors that 
might influence the ultimate results. At the end of the experiment 
observations were made at the outer extremity of the apparent wetted 
area in the original dry part of the box to determine if the advance of 
the moisture had been the same in all parts of the column. In many 
cases it was found that the extent of the movement was a little 
greater upon one side of the column than upon tlie other. These 
differences were probably caused by differences in temperature rather 
than lack of uniformity in packing. 

PROTECTION FROM SUN AND RAIN. 

To protect the flumes from the direct rays of the sun and from the 
rain, canvas covers were provided. These covers were held away 
from the sides of the flumes and from the top by iron bows and iron 
strips similar to the old-fashioned wagon cover. This provided 
ready circulation of air and ample protection from the weather. 
Inasmuch as each flume was protected in this way no corrections had 
to be made for the exposure of the flumes to the sun's rays due to 
differences in angles of inclination or their setting in reference to the 
compass. Figure 1 shows the tank ell or wick and a section of flume 
as they appear when in position for filling. 

THE TANKS. 

The tanks used to contain the water from which the soil columns 
obtain moisture are made of galvanized iron. They are 12 by 20 
inches in area and 8 inches deep. Near the bottom and at one end 
of each tank is fitted a f-ineh water-gage glass, extending upward 
upon the outside of the tank, so that the height of the water in the 



CAPILLARY MOVEMENT OF SOIL MOISTURE, 



11 



tank can be determined after the lid is placed in position. Aronnd 
the outside and at the top of the tank is soldered a galvanized iron 
channel, three-eighths inch wide and three-quarters inch in depth. 
This channel is to receive the edge of the cover to the tank. 

The lid of the tank is of material similar to the tank and has the 
outer edge turned down three-quarters of an inch all the way around 
to fit into th.e channel on the tank. Passinc: throuoh the lid and 




Fig. 1. — Isometric view of open flume toiinected by wick to supply tank. 

soldered to it is the ell. Into the lid is fitted a ^-inch tube through 
which water may be added to the tank. To support the weight of 
the ell and to stiffen the lid, two galvanized-iron channels are riveted 
to the underside of the lid, running crosswise of the tank. These 
channels are placed just outside the ell. 

THE ELL. 

The ell is, as the name implies, an elbow used to change tl\e direc- 
tion of the soil column from the vertical. It extends inches 



12 BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 

within the tank and a few inches within the flume proper. The ell 
is made of galvanized iron and has a cross-sectional area of 100 
square inches. The bottom end of the ell is closed with a piece of 
very fine meshed brass-wire gauze soldered to the ell. The angle of 
the ell is made sufficient to change the direction of the soil column 
from the vertical upward to any specified angle. The angles used 
varied from 45° up to 45° down. 

THE FLUME. 

The flume proper is that part of the equipment designed to hold 
tliat part of the soil column extending beyond the outer end of the 
ell. The bottom and one side of each flume are made of 2-inch red- 
wood plank lined with galvanized iron. The second side of the 
flume is of plate glass, while the top of the flume is open or covered 
Avitli maltoid roofing. The flumes are 10 by 10 inches in area 
and of various lengths. The galvanized lining of the flume at in- 
tervals of 1 foot is ridged or corrugated with 1-inch channels extend- 
ing up and into the flume. The metal lining on the bottom of the 
flume is bent down and over the edge of the plank bottom and then 
bent out and up on the glass side, forming a channel to receive the 
edge of the glass side. This channel is one-half inch wide and three- 
(juarters inch deep. 

THE GLASS SIDE. 

One side of the flume is of stock plate glass cut 11 inches wide 
and 30 inches long. The glass is held in place at the bottom by the 
channel made by extending the lining of the bottom as described 
above. The ends of the glass are held in place by double channels 
made from galvanized iron. These channels are one-half inch in 
width, three-quarters inch deep, and 10| inches long. The channels 
are fastened to the bottom of the flumes by means of screws and are 
held at the top by strap-iron cross-braces fastened to the wooden side. 
IMelted paraffin is run into the channels at the bottom and end of the 
glass and a tight joint secured. The end of the flume is closed with 
a metal gate fastened to the wood of the flume. 

SOIL BOXES. 

The all-metal boxes as first used were replaced with wooden boxes 
having a metal lining. The sizes of the boxes were not altered. 
They are made of 2-inch redwood plank and lined with galvanized 
iron. The lining extends out and over on the open side of the box. 
A strip of plate glass held in place by wooden strips is placed on the 
open side of the box when ready to set in place after packing. The 
wooden strips are fastened to the box proper by means of eyebolts 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 13 

having a screw thread and nut for tightening. The joint between 
the glass and the box is made with a strip of nialtoid roofing. The 
present box gives good satisfaction and is sufficiently rigid to admit 
of considerable handling without danger of cracking the soil column. 

SOIL SAMPLING EQUIPMENT. 

The soil samples are taken with a carpenter's bit, the shank of 
which has been lengthened to 16 inches. The soil samples are placed 
in 4-ounce glass bottles fitted with aluminum screw caps. They are 
dried in the usual double-walled water- jacketed oven. The oven used 
is of local make and of galvanized iron. The inner oven is 12 by 12 
inches, fitted with one shelf. 

EVAPORATION TANK. 

The evaporation tank is made of galvanized iron, and is 18 inches 
square and 12 inches deep. The tank is set in a wooden box 2 inches 
larger all around than the tank. This space is filled wdth soil, thus 
insulating the tank upon the bottom and sides. 

AIR TEMPERATURES. 

The air temperatures are taken with a self-recording thermograph. 
The instrument is set up immediately adjacent to the flumes and is 
shaded and protected from storm. 

ADDITIONAL EQUIPMENT. 

A variety of special equipment has been used, and this will be de- 
scribed with the presentation of the data obtained by its use. 

RATE AND EXTENT OF MOVEMENT OF SOIL MOISTURE BY 

CAPILLARITY. 

There are so many factors controlling the rate and extent of move- 
ment of capillary moisture (4) (11) that it is very difficult to apply 
the data obtained from one soil to a different soil even of the same 
type. Without knowing more of the effects of those different factors 
upon the movement of soil moisture it is not possible to make such 
comparison and expect accurate quantitative results, even though we 
have a complete chemical and mechanical analysis of the two soils 
(8) (15). "Within each soil are those influencing factors, such as 
soluble mineral salts, the organic material, the colloids, and many 
others, which influence in various and irregular ways the movement 
of soil moisture by capillarity. Certain other factors, such as the 
meteorological conditions that may be controlled to a large extent, 
exert a material influence upon the movement of soil moisture by 
capillarity. 



14 BULLETIN 835, U. S. DEPARTMENT OF AGEICULTURE. 

In SO far as the writer Imows, there is very little loiovN-ledge of 
the qua^titati^■e effect of these different factors upon the movement of 
soil moisture, general information being limited to the fact that they 
do infiuencQ the movement. There are a fev>^ data upon the quanti- 
tative effect of temperature (2) and some of the other meteorolcgical 
factors and also of the soluble salts (3), but they are incomplete and 
in some instances confusing. In the experiments herein discussed, the 
evaporation factor has been controlled and taken into account within 
certain limits, and the results of this work will be discussed later in 
the report. 

In any comparison of the data from one soil with the data ob- 
tained from a different soil none of these factors has been taken into 
account. Chemical and mechanical analyses of the soil can be ob- 
tained readily, but with our present knoAvledge such information 
would be of no service in making quantitative comparisons. For in- 
stance : The colloids influence the movement of capillary moisture in 
one wa}^, while the organic material, as indicated by the organic car- 
bon, exerts an influence in the opposite direction. There is not suffi- 
cient information to indicate in the least to what extent these two 
factors might compensate, if at all. Other factors tend to retard the 
movement of the moisture, while others, again, tend to augment it, 
but to what extent our present information does not indicate. 

The experiments herein recorded were run at various times 
throughout the year and in the open. Some of the soils were lested 
during the heat of August and others during the cold weather in 
January. Others of the soils were tested at a time when they en- 
countered periods of almost extreme heat and extreme cold. It is 
know^n with reasonable certainty that the rate and extent of move- 
ment of soil moisture is greater with temperature above but near the 
freezing point than at a higher temperature. That a temperature of 
from' 26° to 32° F. has a marked influence upon soil moisture other 
than the mere fact of freezing will be indicated by data presented 
later in this report. 

In the data herein presented, no corrections are attempted for 
temperature or other factors unless specifically stated. It must be 
kept in mind that in the calculations for comparison and in the 
derivation of formulae the conclusions reached are applicable only 
to the soil under consideration and under the same conditions. 

MOVEMENT OF MOISTURE IN VERTICAL TUBES FROM FREE WATERS. 

The experiments herein recorded differ from other work that has 
been done in vertical tubes only in that the tubes are larger and the 
work has been carried to a greater extent (3), (12). (13), (14), 
These tubes or flumes have also been subjected to variations of tem- 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



15 



peratiire correspondino; to the daily and monthly variations in tem- 
perature of the atmosphere at Riverside. 

A feature of the experiments not nsnally included is a record of 
the (juantity of water required to extend the moisture to various 
lieights. 

Below is given a list of the vertical flumes and the soil placed in 

each. 

Flume 19 was filled with decomposed granite from Riverside, 

Calif. 

Flume 43 was filled with heavy soil from Riverside, Calif. 
Flume 63 was filled wdth heavy clay soil from Whittier, Calif. 
Flume 80 was filled with gravel soil from Uplands, Calif. 
Flume 100 was filled with lava-ash soil from Central Idaho. 
Flume 209 was filled with sandy soil from Central Idaho. 







.' 






y 












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x"'^ t^ o/^ yv\ 




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oVi^^>^\ f^ois. 


in ' 


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f^i-% 


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I' 
















oL. 













1 
















1 












1 1 1 








1 














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. 


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■^ 






































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Ts ^ 










































n'Vf 


^ 








































T5\5^'r 










































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1 




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(y 


( 










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u 


















30 


'if 


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o 


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haht so /J _ 


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N043.ffiVersicls heavy soil _ 
. A"'e3 WhiHier soil 
. NO 80. Uploads soil 

N°IOO, Idaho lova ash SoiJ . 

NO209. Idaho sonc/i/ soil . 


















































1 1 1 




1 1 1 1 1 1 1 1 1 ' 
















1 






n 














1 


1 1 M 1 1 1 M 1 






JO 15 

By hours 



20 24 



20 30 4.0 

By days 



Fig. 2. — Rate of movement of moisture in vertical columnt^ of soil. Tlie numbers within 
circles indicate the point at which that number of liters of water had been taken up. 

The moisture equivalent, in per cent, for these soils is as follows : 
Riverside, light, 7.9; Riverside, heavy, 14.1 ; Wliittier, 38.3; Uplands, 
6.G ; Idaho lava-ash, 18.3 ; and Idaho sand, 4.7. 

Figure 2 shows the curves derived from the measurements of the 
rate of movement of moisture in the flumes and the time of such 
measurements. The vertical element is the distance measured in 
inches and the horizontal element is the time in hours or days. The 
figure to the left shows the rate of movement by hours for the first 
24 hours and the figures to the right the movement by days. 

The curves are parabolos or closely resemble parabolic curves. A 
very rapid movement of the moisture occurs for the first. few hours 
of the experiment. xVfter the first few hours there is a rather rapid 
slowing down of the rate of movement and after about the fifth day 
the rate of movement is rather uniform, growing slightly slower day 
by day. 



16 



BULLETII^ 835, U. S. DEPARTMENT OF AGRICULTURE. 



The diagram indicates that the rate of movement in the lighter soils 
is more rapid for the first few honrs and then slows down much 
quicker than with the heavy soils. The heavier soils maintain a 
relatively more uniform variation than the lighter soils throughout 
the experiment. 

The heavy Idaho soil is an excellent example of those soils having 
a high capillary power. It shows a steady extended movement and 
differs widely from the light Idaho soils, as shown by flume 209. 

We find in these soils a variation of nearly 250 per cent in the total 
distance moved in a period of 30 days. In general, the lighter the 
soil, the shorter the distance the moisture will move upward in a long 
period of time. 

The unnumbered dotted lines upon both the drawings in figure 
2 represent the movement of moisture in vertical tubes of small diam- 
eter as found by Loughridge (13). These curves are introduced to 
show the agreement in results froni experiments with small tubes and 
those from the experiments with flumes at Riverside. The soil used 
in these small tubes, as indicated by the dotted lines, is an alluvial 
soil from Gila Eiver Valley. 

Table 1 gives, in percentages, that part of the distance moved in 
5, 10. and 20 days of the total distance moved in 30 days. 

Table 2 gives the same information in hour periods for the first 
21 hours. 



12 3 



Table L 



-Deiili/ mnroncitt of moisture {<ii^tancc) in pet'ccntngr of movement 
in 30 days. 









Flume. 






Number 
of days. 


























19 


43 


63 


m 


100 


209 


■ 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


1 


51 


41 


47 


46 


30 


62 


2 


61 


60 


57 


57 


43 


71 


3 


67 


66 


62 


62 


51 


76 


5 


74 


73 


65 


70 


59 


83 


10 


84 


82 


76 


80 


77 


89 


20 


94 


93 


89 


92 


92 


94 


30 


100 


100 


mo 


100 


100 


100 



Table 2. — Hoiirlij movement of moisture (distanee) in pereentage of inovement 

in 2Jt hours. 



Number 
of hours. 


Flume. 


19 


43 


63 


SO 


100 


209 


1 
2 
3 

4 
8 
12 
24 


Percent. 
27 
39 

52 
63 
72 

84 
100 


Per cent. 
15 
25 
45 
57 
70 
SO 
100 


Per cent. 


Per cent. 


Per cent. 
14 
29 


Percent. 
44 
58 
69 
80 
81 


29 


38 
56 
66 
71 
83 
100 


47 


55 
63 


83 

100 


74 
100 


90 
100 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



17 



The distcince moved in 2 hours, in percentage of the total distance 
moved in 30 days, or 720 hours, was as follows : 

Flume 19, 20 per cent; flume 43, 13 per cent; flume 63, 13 per cent; 
flume 80, 13 per cent ; Hume 100, 9 per cent ; flume 209, 36 per cent. 
In 2 hours, or 1/360 of the time, the percentage ranges from 36 in the 
light Idaho soil to 9 in the heavy Idaho soil, or from about one-tenth 
to about one-third the total distance. 

About the same relative rate of movement of moisture for the first 
few hours of the experiment is shown b}^ Table 2. In the first 8 
hours, or one-third of the 21 hours, the moisture had moved upward 
more than 70 per cent of the total distance moved in 21 hours, while 
in the longer period of 30 days it is found that more than 80 per cent 
of the total distance was covered in one-third of the time. 

The above tables and diagram but emphasize and give in some de- 
tail the rapid action of capillary moisture when soils are first placed 
in contact with free water, and show that a large part of the total dis- 
tance the moisture will move in a month is covered within the first 
few hours. These results are in accord with those obtained by 
others. (9) 



WATER rSED. 



The figures within the small circles in figure 2 give, in liters, the 
total quantity of water removed from the tanks by the soil columns 
at the end of various periods of time. 

At the end of 30 daj^s, flume 63 had taken about 30 f)er cent more 
water than had flume 19, and flume 100 had taken up nearly twice as 
much water as flume 19. Flume 209 used very little water after the 
first 6 days and a large part of the total water used in the 30 days 
Avas used the first day. At the end of 30 days this flume had used only 
about one-half as much water as flume 19. These figures show, as 
Avas to be expected, that the lighter soils require less water per inch 
than do the heavier soils. 



Table 3. — Quantity of water used to iiwi'e moisture an average distance of 1 
inch at the end of different periods of time. 









Flume. 






Number 
of da vs. 


























10 


43 


(■.3 


80 


100 


2^9 




Liters. 


Liters. 


Liters. 


Liters. 


Liters. 


Liters. 


1 


0.422 


0.446 


0.700 


0. 250 


0.482 


0.296 


2 


.393 


.526 


.7.37 


.256 


.468 


.276 


3 


.332 


.506 


.726 


.323 


..506 


.265 


4 


.379 


.500 


.772 


.311 


.527 


.259 


10 


.366 


.470 


.789 


.326 


.407 


.254 


20 
30 


.364 
.357 


.484 
.472 


.8^0 
.816 


.317 
.327 


.488 
.484 




.239 



147097°— 20— Bull. SS.'J- 



18 



BULLETIN 835, U. S. DEPARTMENT OP AGRICULTURE. 



Except for the lighter soils of the simdj^ type, the (jiiantity 'of 
water required to move the moisture the first mch. is' aboTit -the same 
or a little less than to move it the last inch on a basis of SO-daj' 
tests. Table 3 brings out the fact that there is much less difference 
ill the quantity of water required per inch for the various lieiglits 
than is usually supposed. The difference in the percentage of mois- 
ture found near the bottom and the percentage found near the top 
of a vertical soil column containing capillary water raised from a 
free water surface leads to the natural conclusion that more water 
per iiich is removed for the bottom inches than for the top inches. 
Howe\^r, it is observed that in flume 63 the reverse is true, although 
the percentage of moisture near the bottom of this flume was greater 
than it Avas within 4 inches of the top. Under another heading 
in this report is given at least a pcU'tial explanation of this apparent 
inconsistency. (See p. 56.) 



Table 4.- 



-Watrr removed fro)ii tmik, by days, in percentage of total removed 
ill 30 days. 



Number 
of days. 


riumc. 


19 


43 


63 


80 100 

1 


200 


1 

2 

3 

4 
10 
15 
20 
33 


Percent. 
61 
67 
74 
75 
86 
91 
95 
103 


Per cent. 
47 
66 
69 
72 
79 
86 
91 

100 


Per cent. 
40 
50 
55 
60 
74 
87 
93 
100 


Per cent. 
34 
43 

60 
62 
77 
82 
87 
100 


Per cent. 
30 
42 
53 
60 
79 
89 
92 
100 


Per cent. 
77 
82 
81 

85 






100 



Table 4 shows in general the relatively higli percentages of water 
removed from the tanks during the first day or two and the relatively 
small i^ercentages used after the first three or four days. It is found 
that in all flumes at the end of the third day, or one-tenth of the 30 
days, more than 50 per cent of the water had been used, and by the 
end of the tenth day three-fourths of the total water used in 30 days 
had been removed from the tanks. During the last 10 days of the 
experiment only about 10 per cent of the total water was removed 
from the tanks. 

This table again indicates the longer continued use of the relatively 
large quantities of water by the heavier soils and the very rapid action 
of the lighter soils. This is of economic importance in that the loss 
for an extended time would be much less in proportion for a heavy 
soil than for a light soil where the loss of water is caused by capillary 
action alone. 



CAPILLAEY MOVEMEISTT OF SOIL MOISTURE. 



19 



In Table o ai'e brought together for comparison the rate of advance 
of moisture and tlie quantity of water used, expressed in percentages 
of the totals for 30 days. (See Tables 1 and 4.) 



Tap.t.r n. 



-Prrrmtnfir of distfivcc niored aiifl pcrrpnfnqr of iratcr iiscil hll 
(Idi/s Upon a 30-(l(iy Imsis;. 





Flume. 


Num- 
ber of 
days. 


19 


43 


63 


80 


100 


209 


Dis- 
tance 
moved. 


Water 
used. 


Dis- 
tance 
moved. 


Water 
used. 


Dis- 
tance 
moved. 


Water 
used. 


Dis- 
tance 
moved. 


Water 
used. 


Dis- 
tance 
moved. 


Water 
used. 


Dis- 
tance 
moved. 


Water 
used. 


1 

2 
3 
10 
15 
20 
30 


Per ct. 
51 
61 

. 67 

84 


Perct. 
61 
67 
74 
86 
91 


Per ct. 
41 
60 
68 
82 

■■"93" 
100 


Per ct. 
47 
66 
69 
79 
86 
91 
100 


P€T<A. 

47 
57 
62 

76 

■""89" 
100 


Per ct. 
40 
50 
55 

74 

87 

93 

100 


Perct. 
46 
57 
62 
80 

■""92" 
100 


Per ct. 
34 
43 

60 

77 
82 
87 
100 


Per ct. 
30 
43 
61 

77 

""92" 
100 


Per ct. 
30 
42 
53 
79 
89 
92 
100 


Per ct. 
62 
71 
76 
89 


Per ct. 
77 
82 
84 


94 
100 


95 

100 


94 
100 


"ioo" 



Table 6. — Percentage of 
water, hy volume, con- 
tained in tJie irct so''''. 



It is observed that in three of the flumes the percentage of the 
water used at the end of tlie first day exceeds the percentage of the 
distance moved; in two of the flumes this condition is reversed and 
in the other flume the two percentages are identical. 

Table G shows the j^ercentage of water (b^- volume) contained in 
the wet soil at the end of 30 days, and as would be expected, the rela- 
tively large quantity of water contained in the heavier soils. 

Table 7 gives the depth in inches to which 
the water removed from the tanks at the end 
of the specified time would cover the surface 
of the soil column if none of the water so 
added penetrated the soil. For instance, at 
the end of the third day 671 cubic inches of 
water had been removed from tank 19. This 
quantity of water is sufficient to cover a 10- 
inch by 10-inch area to a depth of 6.71 inches. 
In the same way the other figures of the 
table have been determined. If the area of the flume had been 1 
acre instead of 100 square inches there would have been removed from 
tlie tank sufficient water to have covered the acre to a depth of 6.71 
inches, or, ox]:)ressed in irrigation terms, there would have been re- 
moved from the tank 6.71 acre-inches in the three days. 



Flume. 


Percentage 
of water. 


19 
43 
63 

80 
100 
209 


21.82 
2S. 80 
.30.02 
20. 55 
29.60 
11. m 



20 



BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 



T.'^Bi.E 7. — Dei)th to irhich water removed from tanks icould cover area of 100 

aqxiare inches. 



Number 
of days. 


Flume. 


19 


43 63 


80 


100 


209 


1 
2 
3 
4 

10 
15 
20 
30 
40 
50 
60 


Inches. 
5.49 
6.10 
6.71 
6.86 
7.78 
8.24 
8.62 
9.06 
9.31 


Inches. 
4.27 
6.,10 
6.45 
6.71 
7 42 


Inches. 
4.88 
6.10 
6.71 
7.32 
Q no 


Inches. 
2.44 
3.05 
4.27 

4.47 
5.49 
5.86 
6.24 

7.17 
7.48 


Inches. 
4.88 
6.71 
8. .54 
9.72 
12.81 

14. 34 
14.95 
16.17 
17.39 

15. 37 


Inches. 
2.59 
2.74 
2.82 
2.89 


8. 08 i 10. 62 
8. .54 ' 11. 35 
9.36 1 12.21 
9.70 1 12.57 
10.00 13.06 






3.36 


3.86 






13.42 










1 



Table 7 shows that if a body of water were covered with 40 inches 
of dry soil of the type in flume 100, there would be removed from it 
by capillarity in the first 10 days a depth of 12.81 inches of water. 

If some means were provided to remove from the end of the wetted 
soil column in Hume 100 after the end of the first day all the Avater the 
soil column of this length could transmit, there would be lost from this 
body of water at least 1.83 acre-inches each day, and in one year the 
loss would amount to 55.66 acre-feet per acre. This, then, is the trans- 
mitting power of the soil column at the end of the first day. That 
this amount is not lost each day following the first is clue to the fact 
that the soil can not take this amount by capillarity through the dis- 
tance from the free water. If a calculation were made for this soil 
to transmit water from the twentieth to the thirtieth day and at the 
distance from the water to the outer extremity of the moisture at this 
time, the transmitting power could be 3.17 acre-feet per acre per 
year or only 6.67 per cent of the transmitting power at the end of the 
first day. If the same calculation is made for flume 19, it is found 
that the transmitting power of this soil for the period from the 
twentieth to the thirtieth day is only about one-third that of flume 
100, and about the same relative percentage at the end of the first day. 

In flume 19 it is found that the moisture has traveled upward into 
the flume a total distance of 28.05 inches in three days and that there 
has been removed from the tank at the end of three days a total of 
1,100 cubic centimeters or sufficient to fill the flume to a depth of 
6.71 inches. Using these figures, it is found that at the end of the 
third day there was by volume 23.9 per cent of water in the 28.05 
inches of wetted soil. By the same means of calculation Table 8 is 
computed. 

Table 8 indicates that for a period of 30 days the light sandy 
soils contained a smaller percentage of moisture in the wetted area 
day by day. The heavier soils, as represented by flumes 43, 63, and 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



21 



100, maintain a rather uniform percentage of moisture tliroughout 
the 30 days. (The distribution of this moisture throughout the col- 
umn will be given later.) At the end of 62 days flume 63 contained 
in the wet soil by volume 47 per cent of water, or a little less than afc 
the end of 30 days. At the end of 44 days flume 100 contained by 
volume 29.8 per cent of water, or about the same percentage contained 
for the first 30 days. 
Table S. — Percentage, dy volume, of uater in tvetted soil at end of time specified. 



Number 
of days. 


Flume. 


19 


43 


63 


80 


100 


209 


1 
2 
3 

4 

10 
20 
30 


Per cent. 
25.7 
24.0 
23.9 
23.1 
22.3 
22.0 
21.8 


Per cent. 
27.2 
32.2 
31.0 
30.5 
28.0 
29.4 
28.8 


Per cent. 
42.6 
45.0 
44.3 


Per cent. 
15.3 
1.5.6 
19.9 
19.3 
19.9 
19.4 
20.6 


Per cent. 
30.0 
28.6 
30.6 
32.0 
30.3 
29.0 
29.6 


Per cent. 
18.0 
10.9 
16.1 
15.8 


48.2 
51.7 
50.0 




14.5 



Table 9. — Distrihution of mois- 
ture in vertical soil columns. 



Table 8 would indicate further that capillary action must take 
place much slower in heavj' soils than in light soils, due to the rela- 
tively higher percentage of moisture in the heavy soils at all points 
in the column. It takes a higher relative percentage of moisture 
in the heavy soils to permit the advance of moisture from a damp 
to a dry soil by capillary action. The lighter soils will contain a rela- 
tively smaller percentage of moisture in the very extremity of the 
wetted area as it advances than will the heavier soils. 

DISTRIBUTIOX OF SOIL MOISTUEE IX VEETICAL SOIL COLUMNS. 

The distribution of moisture in a vertical column of soil, the lower 
end of which is in contact with a body of water (d), has received con- . 
siderable attention in these experi- 
ments. The study has included the dis- 
tribution for the various lengths of 
time up to and including 40 days. In 
Table 9 is given the distribution in two 
vertical columns for periods of 30 days, 
or after the column has been in contact 
with the water for a period of 30 days. 
Flume 43 is Eiverside soil No. 1, and 
flume 63 is the "WHiittier soil. Flume 
63 was closed to evaporation, while 
flume 43 was open upon one side, and 
the soil is held in place by brass-wire 
gauze. 

The figures show that the decrease 
in the percentage of moisture from 

the water surface to the upper extremity of the wetted area is 
not uniform. In flume 13, the greatest percentage of moisture is 



Distance 






above 


Riverside 


Whittier 


water 


soil No. 1: 


soil: 


level 


Flume 43. 


Flume 63. 


(inches). 








Per cent. 


Per cent. 


1 


17.40 


46. 74 


3 


17.40 


45.53 





20.44 


40.25 


9 


18.84 


40. 70 


12 


12.07 


40.84 


15 

18 




38.11 
33.49 


12.65 


21 

24 




34.75 

30.82 


12.44 


28 
30 
31 
36 




24.59 


10.15 


5.59 
4.00 


(i. 36 



22 



BITLLETIX 835, V. S- DEPARTMENT OF AGRICULTURE. 



found ut a height of C> inches al)o^ e the water. In Hume 63 there 
is a greater percentage of moisture in the twelfth inch than in either 
the sixth or the ninth inches. In both flumes there is a decrease in 
the percentage of moisture with height above the twelfth inch. In 
flume 43 there is a much more constant and uniform percentage of 
moisture from the twelfth inch to near the top of the wet area 
than there is in flume 63. In both flumes, the moisture content breaks 
very abruptly near the upper end of the wet soil and indicates the 
relatively high percentage of moisture necessary to allow the mois- 
ture to move from the wet to the dry soil. 

Other and very much more numerous data show the irregularity 
of moisture distribution in vertical columns even though every pre- 
caution is taken to have the soil uniform in texture and in density. 
A superficial study of these data would indicate that a fonnula that 
would give the distribution of moisture in vertical soil columns for 
a period of 30 days would be more complicated than the formula 
for the movement of moisture. An anahsis of the above statement 
would indicate that the percentage of moisture which will permit 
the advance of moisture from the wet to the dry soil is variable, 
even for uniform temperatures, etc. 

The data for flumes 43 and 63 given above, and numerous other 
data show a distribution of moisture contrary to general supposition. 

That there is a lack of uniformity in the distribution of moisture 
in vertical soil columns has been observed by others (6), (13), 

THE MOVEMENT OF MOISTURE IN HORIZONTAL FLUMES. 

The horizontal capillary movement of moisture within the soil and 
from a body of free water has not l>een studied before to any great 
extent (1"2). 

INIuch of what has been said of the vertical flumes is applicable to 
the horizontal flumes. The chief difference is rather one of degi^eo. 

At the present time there will be discussed only the horizontal 
flumes open on top to evaporation. 

The number of flumes and the soil contained in each is given in 

Table 10. 

Table 10. — ^oil in Jiorlzontal fliiivc!^. 



Niimber 
of fliune. 



20 
31 
50 
70 
90 
200 



Description. 



Decomposed granite soil from Riverside. Calif. 
Heavy decomposed granite soil from Riverside, Calif. 
Heavy clay loam from Whittier, Calif. 
Sand and gravel wash from Uplands. Calif. 
Heavy lava ash from Idaho. 
Light sand soil from Idaho. 



Figure 3' shows the curves derived from the measurement of 
the movement of moisture in the honzontal flumes and the time 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



23 



of such measiirenieiitfe. The vertical element is the distance meas- 
ured from the surface of the water in the tank and the horizontal 
element is the time in days. 

The resulting 
curves for all these 
soils have the para- 
bolic form. Very 
rapid movement of 
the moistures occurs 
for the first few 
days, after which 
the rate of move- 
ment is more uni- 
fornij but it grad- 
ually decreases with 
the lapse of time. 
It is observed from 
the figure that the 
rate of movement of 
moisture in the heav- 
ier soils, as typified 
by the Whittier soil, 
subsides much more 
rapidly than does 
the movement in the 
sandier soils. 

The extent of 
movement of mois- 
ture in these soils is, 
with the exception of 
Idaho lava soil, in 
inverse order to their 
moisture equivalents. 
That is, the Idaho 
soil (sandy) with 
the lowest moisture 
equivalent showed 
the greatest move- 
ment of moisture, 
while the Whittier 
soil with the great- 
est moisture equivalent showed the least movement of moisture. The 
Idaho lava soil in the horizontal flume as in the vertical flume showed 
a greater movement of moisture than the moisture equivalent would 
indicate. 



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Days 


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Fig. 3. — Rate of movement of m.oist.ure i;i horizontal open 
flumes. Figures in circles indicate points at which that 
number of liters of water had iDeen taken up. The 
dotted line for flume No. 71 (covered) is for comparison 
with flume No. 70 (open). 



24 



BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 



The figures within the .small circles give in liters the quantity of 
water removed from the tanks by the soil columns at the ends of 
various periods of time. 

Table 11 gives the percentages of water used in 1, 3, 5, 10, 15, and 
20 days of the total. quantity used in 30 days. 



Table 11. — Wot, 



rctnnred from tanks 1)1/ <l<iys expressed in percentages of 
amount removed in 30 days. 









Flume. 






Niimher 
of days. 
























20 


31 


50 


70 


90 




Per 


Per 


Per 


Per 


Per 




cent. 


cent. 


cent. 


cent. 


cent. 


1 


17 


22 


26 


18 


17 


3 


30 


36 


42 


30 


29 


5 


38 


42 


51 


36 


37 


10 


53 


58 


67 


52 


53 


15 


67 


70 


78 


64 


07 


20 


81 


81 


86 


79 


81 


30 


100 


100 


100 


100 


IOC 



Table 11 shows the relatively great use of water the first few 
days of the experiment. In all cases more than one-half the total 
quantity of water used in 30 days was used the first 10 days or one- 
third of the time. From 17 to 26 per cent of the total quantity used 
in 30 days w^as used the first day and in two-thirds of the 30 days more 
than 80 per cent of the water was used. The lighter the soil the 
smaller the relative percentage of w^ater used the first few days, and 
the heavier the soil the greater the relative use of water during the 
first few days. However, the lighter tlie soil the greater the total 
quantity that will be used in long periods of time. This is the op- 
posite of the conditions with the vertical flumes and is w^orthy of 
note. The heavier the soil the less extended will be the wetted area 
with the lapse of time, which condition is as would be expected. 
That is, a sandy soil or a light soil will '' sub " much farther in a 
horizontal direction than a heavy soil. The results indicate also 
that a heavy soil loses more water through evaporation when the soil 
is 10 or more inches deep than a sandy soil. This can be accounted 
for from the fact that the capillarity of the sandy soil is not suf- 
ficiently great to keep the- surface wetted to the optimum capillary 
capacity for evaporation. It shows also the influence of gravity 
even in these horizontal flumes 10 inches in depth. Table 12 gives the 
percentage of the total distance moved and the percentage of the total 
water used in 30 da3's for dili'erent periods of time. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



25 



Table 12. — DiKUinre morcfl and irate)' nticd, c.rprexscd in percentages of the total 

for 30 days. 



Nuniber 
of <la-s. 


Flume. 


20 


31 


50 


70 


90' 


Water. 


Pis- 
tance. 


Water. 


dis- 
tance. 


Water. 


Dis- 
tance. 


Water. 


Dis- 
tance. 


Water. 


Dis- 
tance. 


1 
3 
5 

10 
15 
20 
30 


Per cent. 
17 
30 
38 
53 
67 
81 
100 


Per cent. 


Per cent. 
22 
36 
42 
58 
70 
81 
100 


Per cent. 
27 
46 
56 
73 
84 
91 
100 


Per cent. 
26 
42 
51 
67 
78 
86 
100 


Per cent. 
28 
45 


Per cent. 
18 
30 
36 
52 
64 
79 
100 


Per cent. 
29 


Per cent. 
. 17 
29 
37 
53 
67 
81 
100 


Per cent. 
21 
34 
44 
62 
74 
84 
100 


33 

42 
64 
69 
82 
100 


52 
70 
81 
88 
100 


69 
80 
87 
100 



The tables show what was to be expected, that relatively more 
water was required to advance the wetted area in the flume 1 inch 
near the end of the experiment than at the commencement of the ex- 
periment. This fact may be partially or wholly due to the loss of 
water by evaporation. However, from data secured with the vertical 
flumes it may be partially due to changes in the distribution of mois- 
ture through the length of the flumes. 

Table 13 gives the average quantity of water, in cubic centimeters, 
required to advance the moisture in the flume 1 inch for different 
periods of time. 



Table 13. 



.\rrn([/e ii-^e of water to adranee the moisture 1 inch for different 
periods of time. 



Number 
of days. 


Flume. 


20 


31 


50 


70 


90 


200 


1 
3 
5 
10 
15 
20 
30 


c. c. 


c. c. 
500 
474 
461 
488 
501 
552 
616 


c. c. 
711 

728 


c. c. 
259 


c. c. 
524 
532 
537 
545 
577 
617 
637 


c. c. 
300 
290 
305 


380 
384 
400 
413 
422 
425 


288 
300 
320 
366 
408 


764 
775 
778 
789 


345 
380 





Since these flumes are open to evaporation, more water is required 
to advance the moisture an inch as the distance from the tank 
increases. In some instances the amount of moisture required to 
advance the wetted area 1 inch at the end of 30 days is nearly double 
that required the first few days. In the heavier types of soils, as 
represented by flume 50, a more constant quantity is required during 
each of the 30 days than for the lighter soils as represented by flume 
70. Table 13 indicates also that less w\ater is required to advance 



26 



BULLETIN 835, IT. S. DEPARTMENT OF AGRICULTURE. 



the moisture an inch in light soils than in heavy soils. In flume 31 
more moisture was required per inch of advance during the first few 
days than during the fifth day, but after the fifth day there was a 
gradual increase in the moisture required. This same condition was 
found in flume 200. It is probable that this results from the fact that 
the moisture percentage changes to a very much greater extent near 
the tank end of the flume than it does toward the other end, and 
especially is this true the first few days. It is also noted from the 
results of the vertical flumes that for a distance of 14 inches above 
the surface of the water the moisture moves rather slowly upward. 
It is probable, therefore, that during a period along about the fifth 
day there is not sufficient moisture near the top of the flume to permit 
a maximum e^'aporation. After that time evaporation takes place 
more rapidly and hence the increase in water consumed. Another 
fact that will be brought out in the distribution of the soil moisture 
in these flumes is the gradual increase in the percentage of moisture 
throughout the wetted area of the flume from clay to clay, this con- 
stantly increasing percentage continuing until very near the point of 
capillary saturation. 

In a review of Table 13 it is found that the order of the Avater 
requirements of these flumes at the end of the twentieth day, begin- 
ning at the one requiring the least water, is flume 70, 200, 20, 31, 90, 
and 50. Comparing this order with the order of the moisture equiva- 
lents of these soils, and beginning with the' least moisture equiva- 
lent, we find the order just the same as above, except that flumes 70 
and 200 are reversed. This interchange of place is probably ac- 
counted for by the fact that flume 200 would permit of more evapora- 
tion per square inch than would flume 70. 

From the data in Table 13 it is possible to calculate for the hori- 
zontal flumes the quantity of water removed during any period of 
time, in cubic inches. Assuming the same rate of use in nature as in 
the flumes, the use of wateT by the soil in place can be calculated in 
acre-inches. In preparing Table 11 such assumptions were made. 



Table 14. — Loss of loatcr from tanls in different periods of time, 
pressed in indies on an area of .100 square inches. 



{Depth ex- 









Flume. 






Number 
of days. 


























20 


31 


50 


70 


90 


200 




Inches. 


Inches. 


Iiiches. 


Inches. 


Inches. 


Inches. 


1 


4. .'i8 


6.10 


4. 88 


3.67 


7.32 


6.10 


3 


7.93 


9.76 


7.93 


6.10 


12.21 


9.15 


5 


10.07 


11.59 


10.76 


7.34 


15. 87 


11.. 59 


10 


14.34 


15.87 


12.81 


10.37 


22.58 


15. 25 


15 


18.00 


18.22 


14.95 


12.82 


28.68 


19.53 


20 


21.82 


22.27 


16. 48 


15.87 


34. 78 


21.40 


30 


26.85 


27.46 


19.12 


20.14 


42.72 







CAPILLARY MOVEMEiSTT OF SOIL MOISTURE. 27 

Table 14 is interesting from the fact tliat at the end of the first 
few clays the use of water by the flumes containing the lighter soils 
is greater than for the flumes containing the heavier soils. The use 
of water by flume 50, containing the Whittier soil, is, after the first 
few days, considerably slower than that by flume 200, containing the 
light sandy Idaho soil. This fact is of importance and confirms the 
observations in nature of the excessive loss by capillarity in convey- 
ing channels constructed through sandy soils. This table, in con- 
nection Avith figure 3, indicates the extensive and long-continued 
capillary action in a horizontal direction in the lighter soils. 

DISTRIBUTION OF MOISTURE IN HORIZONTAL FLUMES. 

In considering the distril)ution of moisture in horizontal flumes 
open on top to evaporation, it is difficult to obtain uniform comparable 
results. This is due to the fact that the flumes were exposed to the 
natural changes of meteorological condition and many of them were 
in oi:>eration during the extremes of temperature. Another fact .that 
is of primary importance is the effect of temperature upon the vertical 
distribution of moisture within the flume. With temperatures near 
the freezing point and with the soil containing about its maximum 
capillary capacity of moisture, a distribution of moisture is found in 
the soil difl'ering materially from the distribution in the same soil 
with higher temperatures. It is not thought, therefore, of value in 
presenting a few data to attempt any specific calculations, but only 
general comments are made. 

In Table 15 the first column gives the date on which the sample 
was taken; the second column gives the distance along the top of the 
flume, measured from the intersection of the top line of the' flume and 
a vertical extension of the inside of the vertical part of the wick. 
This point is 19:^ inches above the water surface, measured along the 
upper side of the wick. The third column gives percentages of 
moisture at the various points for the top 5 inches of the flume, and 
the fourth column for the bottom 5 inches of the flume. The fifth col- 
umn gives the average percentages of moisture at the various points. 

Taking the average percentages of moisture in flume 31 at the same 
point and on different dates, it is found that the percentage of mois- 
ture gradually increases until the warmer weather in June. After 
that time there may be a slight decrease in percentages of moisture 
at the different points. Taking a sample at the 9-inch point, we find 
this to be true and that the percentage of moisture on June 10 had 
decreased about 2.2 per cent from what it was on May 23. Compar- 
ing the percentage of moistures for the top 5 inches of soil at the 
9-inch point, we find that throughout the entire time there was a 
gradual increase in the percentage of moisture, while' the bottom 5 
increased in moisture content until April and then decreased. 



28 BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 

Table 15. — Distribution of molnture in horizontal flumes. 



FLUME 20. 



Date. 



Apr. IS. 



Dis- 
tance. 



Incites. 
3 
9 
21 
45 
69 
SI 
93 
105 
111 
117 



Sept. 21 



Top 5 
inches. 



Per cent., 
25.72 
20.21 
19. SO 
17.00 
15. 74 
12.00 
13. 34 
11.52 
10.30 
8.36 



Bottom 
5 inches. 



Per cent. 
23.04 
21.45 
21. S2 
19.44 
17.66 
16.47 
15.03 
13. 21 

m. 7s 

6.76 



Average. 



FLUME 31. 



Apr. 20.. 


9 


21.20 


22.96 




15 


20. 42 


21.66 




21 


IS. 99 


20.00 


Apr. 29.. 


9 


23.02 


25. 49 




15 


23.02 


24. .52 




27 


21.82 


22.96 




33 


IS. 98 


21. S4 


May 23.. 


9 


23.66 


25.07 




33 


20.39 


22.76 




51 


18.45 


20.18 




59 


16.43 


17.76 


Jiuie 10 . 


9 


23.72 


• 21.98 




22 


22.55 


23.65 




34 


19.77 


22.74 




52 


18. 35 


20.64 




64 


13.81 


17.88 



FLUME 50. 



3 


43.31 


45.86 


12 


44.50 


42.52 


24 


39.16 


41.82 


30 


3S.92 


39.94 


33 


3.5.65 


37.01 



Per cent. 
24.38 
20.N.5 
20.81 
18.22 
16.70 
14.24 
14.18 
12.36 
10. .54 
7. .56 



22.12 
21.04 
19. .50 
23.75 
22.39 
22.-39 
20.41 
24.36 
21.57 
19.31 
17.09 
22. .S5 
23.10 
21.25 
19.50 
15.85 



44.58 
43.61 
40.49 
39. 43 
36.33 



FLUME 70. 



Apr. 7. 



Date. 


Dis- 
tance. 


Top 5 
inches. 


Bottom 
5 inches. 


Average. 


Oct. 14.. 


Inches. 

3 

30 


Per cent 
13. .so 
4.59 


Per cent. 
14.60 
9.62 


Per cent. 
14.20 
7.10 


FLUME 90. 



Jan. 29.. 


J 


2S. SO 


2S. SI 




18 


26.37 


26.66 




42 


19.30 


20.07 


Feb. 29.. 


§ 


30 53 


29.15 




18 


2S.44 


27.87 




42 


25.44 


25.04 




72 


18. SI 


19.75 


Feb. 22.. 


i 


32.30 


29. ,S3 




9 


30.22 


27.49 




18 


28. 35 


26.17 




30 


26.76 


26.05 




42 


25. S6 


25.27 




54 


25. 35 


25.58 




72 


17.65 


23.33 




84 


22.35 


22.80 




96 


18. 53 


19.87 


Mar. 5... 


I 


31.53 


29.. 52 




IS 


27.44 


26.30 




42 


26.33 


25.06 




72 


23.37 


23.67 


Mar. 10.. 


371 


26.50 


20.00 



FLUME 200. 



28. SO 
26.52 
19.69 
29.84 
28. 15 
25.24 
19.28 
31.07 
28.85 
27.21 
26.40 
25.57 
25.47 
20.49 
22.57 
19.20 
30.52 
26.87 
25.99 
23.52 
26.25 



3 


11.92 


16.28 


4 


12.50 


IS. 25 


2s 


10 37 


11.95 


46 


14.61 


16.71 


64 


9. .59 


13.00 


End. 


4.43 


5.02 



14.10 
15.37 
11.34 
15.66 
11.30 
4.72 



111 flume 00 the same conditions are found as in flume 31, except 
that during the months of January and February there is an appar- 
ent discrepancy in the percentages of moisture in the bottom 5 inches 
and the top 5 inches of soil This apparent discrepancy is probably 
the result of temperatures below the freezing point and will be 
considered in a subsequent part of this report in connection with 
other similar analyses. 

All of the flumes show a gradual decrease in the percentage of 
moisture from the tank end of the flume to the outward extremity 
of the wetted area= In flume 20, the average percentage of moisture 
decreases at the rate of approximately 1.75 per cent for each linear 
foot between the third and eleventh feet. In flume 50 the rate of 
decrease is about 2.2 per cent per linear foot. The rate of decrease 
varies in these flumes, as would be expected not only from the dif- 
ferent meteorological conditions when the experiments were run, but 
also from the character of the soil. 



CAPILLARY MOVEMENT OF SOIL MOISTITRE. 



29 



FLUMES INCLINED DOWNWARD FROM THE HORIZONTAL. 

The flumes in which it was intended that gravity should assist the 
capiHary movement of moisture Avere inclined downward at various 
angles from the horizontal. In all the flumes inclined in this way 
the movement of the moisture and the amount of water used were 
greater than for the horizontal flumes or the flumes inclined upAvard 
from the horizontal. The extent to which water would move in the 
inclined flumes where the inclination downward was 10 degrees or 
more was, in most cases, bej^ond the limits of the equipment used. 
j\Iost of the experiments were carried to such an extent as to warrant 
certain conclusions. The extent of this movement in the open flumes 
appears to be limited not by the friction factors, but liy the jiower of 
the wick to supply moisture in sufficient quantity to take care of 
the evaporation from the flume. That is, were evaporation elimi- 
nated, the extent of movement in the flumes inclined downward at 
angles greater than 30 degrees, except for the very heavy soils, would 
be far beyond experimental limits. In the case of the very heavy soils, 
as typified b}" the Whittier type, there were indications that in the 
less steeply inclined flumes friction played its part here as well as 
in the horizontal flumes. 

In distribution of moisture there are found some differences be- 
tween these flumes and either the vertical or the horizontal ; and, as 
wdll be shown later free water was developed in the flumes inclined 
downward. 

SOILS X'SED. 

Table 16 gives the numbers of the flumes and the soil contained in 
each : 

Table 16. — Soils in flumes inclined doicnicfird. 



Number 
of flume. 



4 

3-i 
54 
74 
94 
204 



Description. 



Decomposed granite soil from Riverside, Calif. 
Decomposed granite and clay from Riverside, Calif. 
Heavy clay soil from Whittier, Calif. 
Sand and gravel soil from Uplands, Calif. 
Lava ash from Idaho. 
Sandy soil from Idaho. 



Figure 4 gives the dates and measurements for the movement 
of moisture in flumes inclined at an angle of 30 degrees and open on 
top to evaporation. The horizontal element is the time and the 
vertical element the distance in inches. 



30 BULLETIIsr 835, U. S. DEPARTMENT OF AGEICULTURE. 

A comparison of figui^e 4 with figure 2 shows very strikingly the 
part gravity plays in capillarity. It shows to what extent gravity 
aids or retards the movement of soil moistm'e by capillarity. An- 
other striking feature is the comparative uniformity of the rate of 
movement of the moisture after the first three or four days. Wliile 
there is a general slowing down of the rate at which moisture ad- 
vances from day to day, it is so much less marked in these flmnes 
than in the flumes discussed in previous sections as to be of compara- 
tively little moment. 

It is observed that after the first day or two the type of soil used in 
the flumes is of greater importance in limiting the extent of the move- 
ment of the moisture. The more open and porous the soil, the more 
rapid and extended the movement of the moisture. For instance, in 
the sandy Idaho soil of flume 204, the moisture advanced as far in 
one day as it would in the heavy Riverside soil in five and one-half 
days and 50 per cent farther in the first day than it would in the 
heavy IVliittier soil in 30 days. In flume 204 the only limit to the 
extent of the movement of moisture was the ability of the wick to 
furnish the moisture. However, the porosity of the soil is not the 
only factor, but the transporting power of the soil itself is of prime 
importance. For instance, comparing flume 34 (heavy Eiverside) 
with flume 74 (Upland), fiume 34 has the greater rate of movement 
of moisture at all times within the limits of the experiment, and yet 
the soil of flume 74 has the greater porosity. The difference in the 
rate of movement in these two soils appears to be due to the difference 
in the capillary power of the wick to transmit the water from the 
tanks to the flumes proper. Had there been less vertical lift from the 
tank to the flume by the wick, flume 74 would undoubtedly have 
shown the greater rate of moisture movement. The effect of porosity 
is well illustrated in flumes 74 and 94. The soil in flume 94 has the 
greater porosity, and while the rate of movement of the moisture is 
less in this flume for the first week, it has the greater rate of move- 
ment thereafter. Again, comparing flumes 4 and 34, the soil in flume 
4 has the greater porosity, but the soil in flume 34 the greater capil- 
lary power, and after the first two weeks the rate of movement of 
moisture in flume 34 is greater. 

In table 17 is given the extent of movement of moisture as shown in 
figure 4, in percentages of the extent of movemicnt in flume 34. 

That is, in flume 34 the moisture had moved the first day 26 inches, 
or 100 per cent. In flume 4 the moisture had moA^ed the first day 
28.85 inches, or, as compared with the movement of moisture in flume 
34, 111 per cent, while in flume 54 the moisture had moved 10.7 
inches, or, based on the movement in flume 34, 41 per cent. Flume 34 
maintained a relativelv higher rate of movement of moisture than 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



31 



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LL______ E ~ — —C — 



OS 
Of 

oq 

09 
OL 
08 
06 

001 5- 

o 

0/( ui 

OZl 

OSI 

Ot'I 

051 



0L\ 



02\ 



061 



Fig. 4. — Rate of movement of moisture iu open flumes inclined downward at tbii-ty degieei 
from tlie horizontal. Figures iu circles indicate points at which that number of liters 
of water had been taken up. 



32 



BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 



Table 17. — Comparative movement of moisture, 
in percentages of movement in flume 34- 



any other flumes, although flume O-t maintained nearly the same rate 
after the first day. In view of the fact that these flumes were oper- 
ated at different seasons of the year, it is not possible to say to what 

extent the variable me- 
teorological conditions 
might have influenced 
the results. 

In Table 18 there is 
shown for each flume 
the percentage of the 
total distance moved in 
30 days that had been 
moved in 1, 3, .5, 10, 15, 
and 20 days. 

Table 18 shows in an- 
other way what has been 
previously stated. The heavier soil and less porous soils show a rel- 
jitively greater percentage of movement of moisture the first day or 
two and a relatively slower rate of movement the last few days. The 
lighter and more porous soils show the more uniform and more ex- 
tended movement of the moisture. It is found that in all the flumes, 
in 5 days, or one-sixth of the 30 days, more than one-third of the 
total 30-day distance was traveled; in 10 days, or one-third of the 
time, more than one-half the distance has been traveled, and in 20 
days, or two-thirds of the time, more than four-fifths the distance 
has been covered. 

In the discussion previously given of these flumes onlj^ the 30-day 
limit of time was used. However, in figure 4 the curve for flume 









Flume. 






Number 
of clays. 
























4 


34 


54 


74 


94 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


1 


Ill 


100 


41 


85 


82 


3 


111 


100 


42 


87 


85 


5 


108 


100 


39 


87 


87 


10 


102 


100 


34 


79 


S3 


15 


99 


100 


31 


78 


78 


20 
30 




100 
100 


29 

27 


78 
75 


78 
80 







Table IS. — Movement of moisture 'by 
(lays, ill percent ages of total move- 
ment in 30 days. 



54, the heavy Whittier soil, 
shows that after 30 days the 
rate of movement of the mois- 
ture continues to grow less and 
le^s every day, although there is 
considerable uniformity in the 
rate of decrease of movement. 
The figures would indicate that 
the movement of moisture 
would reach a considerably 
greater distance than that 
shown upon the figure. It is 
seen that in flume 74 (Upland soil) after 47 daj^s the rate of movement 
of the moisture is not much less than it was at 30 days, and the evi- 
dence is that the moisture would continue to move in this flume rather 
indefinitely ; especially would this be true were evaporation prevented. 





. 


Flume. 




Number 
of days. 










1 






34 


54 74 


94 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


1 


14 


22 


16 


15 


3 


28 


37 


■29 


26 


5 


32 


46 


37 


35 


10 


50 


62 


53 


52 


15 


65 


74 


68 


65 


20 


79 


85 


82 


80 


30 


100 


100 


100 


100 



CAPILLARY ^MOVEMENT OF SOIL MOISTURE. 



33 



wati:k x\si-:i). 



Table 19. — Water used, hy days, In 
pcrcentaycs of total use in 30 days. 



The figures in the small ciiclcs show in liters the water nsed by 
these flumes. The water used b}^ flumes inclined downward is, like 
the movement of the moisture, greater than for tlie horizontal or 
vertical flumes. A striking feature is the rather uniform use of a 
comparati\ely constant quantity of water after the second or third 
day. The rate of use is more constant and uniform than for the 
vertical or horizontal flumes. Flume 34 had used on the tenth day 
about 4 liters of water; on the twentieth day about 8 liters; on the 
thirtieth day about 11.5 liters; and on the fortieth day about 14 
liters. In flume 54 with the heavy "VVliittier soil an even greater 
uniformity is observed. In this flume there w^as used approximately 
the same quantity of water every day after the sixth day up to the 
fifty-seventh day, or at the end of the experiment. The same 
uniformity is found, in fact, in 
nearly all of the other flumes. 
One fact worth special notice is 
that the use of water by the flume 
as represented by the loss of 
water from the tanks is that 
evaporation does not appear to 
have varied the use to any ex- 
tent. This is true though the 
same flume was exposed to al- 
most all the different and vari- 
able weather conditions found at 
Riverside. To show the relative 
uniformit}^ in the rate of use of water b}^ some of these flumes 
Table 19 has been prepared. 

Table 19 shows that the heavier soils use relatively more water 
at the commencement than near the end of the experiment. It shows, 
also, a more uniform use by the heavier soils. It shows, for instance, 
that the soil in flume 54 had used relatively more than twice as much 
water as any other flume at the end of the first day, while on the fif- 
teenth day it had used relatively only about one-fifth more than the 
others. Table 20 shows the amount of water required at different 
periods of time to advance the moisture in the flumes an average dis- 
tance of 1 inch. For instance, on the third day, flume 24 had used 
18 liters of water and the moisture had advanced 44.15 inches, or an 
average of 479 cubic centimeters of water was required per inch. 

A comparison of the figures in Table 20 with the moisture equiva- 
lents of the soils appears to show no close relation. However, in a 
general way the greater the moisture equivalent the greater the 
(luantity of water required to advance the moisture 1 inch. It is ob- 
147697°— 20— Bull. 835 ?> 







riu 


m.^. 




Numfce;- 
of daj-s. 




















?A 


:4 


74 


'.:i 




Pcrce-t. 


PcT c:-J,. 


Percent. 


PlTCC^t. 


1 


9 


22 


10 


11 


3 


18 


2 ) 


19 


20 





23 


43 


25 


28 


10 


39 


5; 


41 


43 


15 ■ 


."4 


C8 


0) 


57 


20 


71? 


',8 


C8 


70 


30 


100 


100 


100 


100 


40 

50 


121 


127 
149 


129 








' ""i 



34 



BULLETIX 835, U, S. DEPAKTMENT OF AGRICULTURE. 



served in nearly all of the flumes that less ^vater is required per inch 
about the third day than at any other time. In all cases, however, 
more water was required per inch at the end than was required at the 
beginning of the experiment. It is observed that for soils of the 
heavier type represented in flume 54, for some time after the com- 
mencement of the experiment less water is required per inch than 
for the following day, but after about the thirtieth day there is a 
very rapid increase of the water requirements. It is probable that 
there is some concentration of moisture at the top of the vertical lift 
before the moisture changes direction to the inclined part of the flume 
and that this moisture is partiall}^ drawn upon to advance the mois- 
ture in the inclined part of the flume. After a few days this surplus 
supply, if such it may be called, is exhausted and then the moisture 
to advance the wetted area in the flume can be derived only from, the 
supply in the tank. It must be kept in mind also that with the lapse 
of time a greater wetted area is exposed to evaporation, and this in 
itself would account for some additional water requirement per indi. 
In some' cases the water requirement per inch at the end of the for- 
tieth day was about double the requirement the first day, but in the 
heavier soils this is not so pronounced. 



Table 20.- 



-Watcr used to advance moisture 1 inch at different times, in ctiMc 
centimeters. 



Number 
of days. 


"lume. 


4 


34 


54 


74 


94 


294 


1 
3 
5 

10 
15 
20 
30 
40 
50 
57 


c.e. 
319 
346 
425 
450 
545 


c. c. 

3S5 
447 
498 
533 
5H9 
007 
684 


c.e. 
743 
707 
700 
677 
CSO 
097 
735 

soa 

846 

884 


c.e. 
290 
338 
336 
364 
411 
419 
507 
567 


C. C. 
566 
562 
671 
597 
634 
647 
724 


e.c. 
3U 
360 











































PLUMES INCLINED UPWARD FROM THE HORIZONTAL AT AN ANGLE OF 15°. 

To throw some light upon the effect of a relatively small inclina- 
tion of the flumes upward from the horizontal, the data will be given 
and discussed for the flumes inclined upward at an angle of 15° and 
open on top to evaporation. The flumes are the same in every respect 
as the others, except the angle of inclination. In these flumes there 
is a vertical lift of 4 inches ]>eiore a change is made in the direction 
of the flumes. 

They show a much less movement of the moisture and a much less 
use of water than the horizontal flumes, but a mOre extended mo^'e- 
ment of the moisture and greater use of water than the vertical 
flumes. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



35 



SOILS \'SED. 

Table 21 gives a list of the flumes inclined upward at an angle of 
15° and soils contained in each. 

Table 21. — Soils in flimies inclined itpicard at an angle of 15°. 



Number 
of flume. 



39 

58 
76 
96 
206 



Description. 



Decomposed granite with clay from Riverside, Calif. 
HeaA^y clay soil from Wluttier, Calif. 
Sand and gravel soil fi'ora Upland, Calif. 
Lava ash from Idaho. 
Sandy soil from Idaho. 



MOVEMENT OF MOISTURE. 

FigTire 5 gives tlie distance the moisture had moved in the flumes 
at the end of the time indicated. The horizontal element is the time 
in days and the vertical element is the distance in inches. 




Fig. 5. — Rate of movement of moisture in open flumes 
inclined fifteen degrees upward from the horizontal. 
Figures in circles indicate the points at which that 
number of liters of water had been taken up. 



36 



BULLETIN 835, IT. S. DEPARTMENT OF AGRICULTURE. 



From li^ni'G 5 it is seen that the curves for the movement of mois- 
ture have the same parabolic form as the curves in the preceding 
figures. A comparison of these curves with those for the vertical and 
horiz(mtal flumes shows the importance of gravity in the rate and 
extent of movement of moisture by capillarity. 

The curves show that the rate of movement of moisture is rather 
more uniform over an extended period than in the vertical flumes. 

After the first two or three davs 



Table 22. — Extent of moisture move- 
ment in ffiimes at rarious times. 



Number 
of days. 


I'lume. 


31 


ns 


70 


96 


1 
3 
5 
JO 
15 
20 
30 
40 
50 


Pi r cent. 
34 
51 
5'l 
74 
83 
90 
100 
106 
112 


Per cent. 
30 
53 
.59 ■ 
75 
84 
91 
100 
108 


Per cent. 
30 
53 
61 
75 
84 
90 
100 
106 
109 


Per cent. 
21 
37 
48 
66 
78 
86 
100 









there is a gradual slowing down 
of the rate of movement from 
day to day. Where the experi- 
ment is carried on for 50 days or 
more it is observed that the rate 
of movement is very slow at that 
time. 

Table 22 gives the extent of the 
movement of the moisture at 
various times, in percentages of 
the movement in 30 days. 
It is observed from Table 22 
that the relative rate of movement in the first three flumes day by 
day Avas about the same. In flume 96, however, the rate of movement 
of the moi.sture Avas relatively not so great during the forepart of 
the experiment, but that a more uniform rate cf movement was 
maintained throughout. In the first three flumes more than onedialf 
of the total 30-day distance had been traveled in three days, or one- 
tenth of the time, and in two-thirds of the time more than nine- 
tenths of the 30-day dis- oo « t ,• , ^ • , 7 

, T -, Table 23. — Relatire movement of moisture, hij 

tance had been traveled. percentage of movement, in flume 96. 

In flume 90 on the third 
day only about one-third 
of the distance had l)een 
traveled, and it was not 
until aliout the sixth day 
that one-half of the dis- 
tance had been traveled. 

From Table 23 it is 
found that on the thir- 
tieth day the moisture 
in flume 58 had moved but -fO per cent as far as in flume 90, while in 
flume 39 the moisture had moved one-half as far as in flume 90. 

All of these flumes when compared with flume 90 shoAV a lesser 
relative movement during the latter part of the experiment than 
during the forepart of the experiment. This table shows also that 
the heavy soil as represented in flume 58 has a much less rapid rate 
of movement during the forepart of the experiment, but that the 









Flume. 






Number 
of days. 
























39 


58 


76 


96 


206 




Percent. 


P(r cent. 


Percent. 


Percent. 


Percent. 


1 


80 


60 


90 


100 


117 


3 


69 


56 


91 


100 


90 


5 


62 


50 


82 


100 


80 


10 


67 


46 


73 


100 


r,6 


15 


54 


44 


70 


100 


60 


20 


53 


43 


68 


100 


57 


30 


50 


40 


65 


100 







CAPILLARY MOVEMENT OF SOIL MOISTURE. 



37 



rate of iii.ovenient as compared with flumes 39 and 70 is more uni- 
form. The figures for flume 206 show the rapid decrease of rate of 
movement of the moisture from day to day. 



WATER USED. 



Tabee 24. 



Wafer used by flumes at different 
periods of time. 









Flume. 






Number 
of days. 
























39 


5S 


76 


96 


206 




Liters. 


Liters. 


Liters. 


Liters. 


Liters. 


1 


7.5 


10.5 


5.0 


11 


7.0 


3 


13.0 


15.0 


8.0 


20 


9.0 


5 


15.5 


19.0 


10.0 


27 


10.5 


10 


19.0 


23.0 


14.0 


39 


13.5 


15 


22.0 


25.5 


17.0 


48 


14.75 


20 


24.5 


28.0 


20.5 


53 


15.75 


30 


27.5 


32.5 


26. 25 


65 




40 
50 


31.25 
35.0 


36.75 
41.0 


33.0 
40.0 















The amount of water used in the flumes inclined upward is greater 

than for the vertical flumes and less than for the horizontal flumes. 

The total quantity of 

water used by the flumes 

inclined at an angle of 

15° upward from the 

horizontal is given in 

liters in figure 5 in the 

small circles. Table 24 

gives the total quantity 

of water, in liters, used 

by the different flumes 

at the end of different 

periods of time. The 

same information i s 

given in Table 25, in percentages based upon the quantity of water 

used by each flume at the end of the thirtieth day. 

This tal)le shows that the heavier soils as represented in flumes 39 

and 58 use relatively more water during the first few days of the 

experiment than do the lighter soils. In the heavier soil about the 

fourtli day 50 per cent of the total quantity of water used in 30 days 

had been used, while for the 
ligliter soils there had been used 
on the fourth day only about one- 
third the quantity used in 30 
days. 

Table 26 gives the average 
quantity of water removed from 
the tanks at different periods of 
time to advance the moisture in 
the flumes an average distance of 
1 inch. That is, in flume 39 at 
the end of the fifth day there had 
been removed from the tank 15.5 

liters of water and the moisture had advanced in the flume a total 

distance of 28.55 inches, or there had been used an average of .543 

cubic centimeters of water for each inch the moisture had advanced. 
Table 26 shows that in the lighter soils the quantity of water used 

near the beginning of the experiment was very much less than the 

quantity of water used during the last part of the experiment. In 



T.\BEE 25. — Water used, hy days. J7i per- 
ceutaycs of total used in 30 days. 







Flume. 




Number 

of (Says. 


















39 


5S 


76 


96 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


1 


27 


32 


19 


17 


3 


47 


41 


31 


31 


5 


57 


59 


38 


41 


10 


69 


71 


53 


60 


15 


SO 


78 


64 


74 


20 


90 


86 


78 


86 


30 


100 


100 


100 


100 


40 
50 


114 

121 


113 
126 


125 
152 






1 



8« 



BULLETIN 835, U. S. DEPAKTMENT OF AGRICULTURE. 



Table 


26. — Wafer use 


I per inch of ad 


vanrc. 








Flume. 






Number 
of days. 












39 


58 


TTj 


96 


206 


1 
3 
5 
10 
15 
20 
30 
40 
59 


c.c. 
406 
530 
543 
531 
656 
573 
518 
605 
686 


c. c. 

816 
835 
826 
786 
782 
792 
840 
891 


c. c. 

272 
215 
266 
302 
327 
366 
423 
487 
589 


c. c. 
541 

558 
589 
617 
644 
679 
676 


c. c. 

297 
278 
290 
325 
301 
348 















flume 76 at the end of the fiftieth day there was used twice as much 
water per inch as for the first day. In flume 39 there is shown after 
the third day somewhat of an increase in the use of water from day 
to day, but it is much less marked than in any of the other flumes. 
In flume 96 the use of water on the thirtieth day is about 25 per cent 
in excess of the ase on the first day. The increase in the quantity of 

moisture required per 
inch witli the laj^se of 
time is p r o b a b 1 y due 
largely to the effect of 
evaporation. In flume 
58 the distribution of 
moisture was so uniform 
as compared with the 
other flumes that the 
quantity of water in the 
flume per inch through- 
out its length is almost 
the fjame, with the exception of the upper few inches. In the 
other flumes there is a marked decrease in the percentage of moisture 
from near the tank to the outer extremity of the flume. The relation 
of the figures in this table to each other corresponds very closely with 
the relation of the moisture equivalents for the soils represented. 

To show the amount of water removed from the tanks b}' the 
flumes expressed in depth in inches on an area equal to the cross 
section of the flumes. Table 27 is presented. 

At the end of the thir- 
tieth day it was found 
that the flumes had taken 
from the tanks sufficient 
water to cover the cross 
section of the flumes to a 
depth of from 16 to 40 
inches. That is, where the 
rate of loss is the same 
over the area of an acre 
as over the area repre- 
sented b}^ the flumes, 
then in 20 days the acre 
of soil represented in flume 39 would have removed from the under- 
ground water 16.78 acre-inches of water, while the soil represented 
])y flume 96 would have removed 39.65 acre-inches of water, or a 
little more than twice as much. These tables are valuable in that 
they give an indication of the quantity of water that may be removed 



Tabu. 27. — Water rcDiored from the tanJ:.^ hy 
capUlaritu expressed in depth on an area equal 
to the cross section of the flume. 









Flnme. 






Number 
of days. 


39 










58 


76 


96 


236 


1 

3 
5 
10 
15 
20 
30 
40 
50 


Inches. 
4.58 
7.93 
9.44 
11.59 

13. 42 

14. 95 
1-6.78 
19.06 
21.35 


Inches. 
6.41 
9. 15 
11.59 
14.03 
15. 56 
17. 08 
19.83 
22.42 
25.01 


Indies. 
3.05 
4.88 
6.10 
8.54 
10.37 
12.51 
15,99 
20. 13 
24.40 


Inches. 
6.71 
12.20 
16.47 
23. 79 
29.28 
34. 16 
39.65 


Inches. 
4.27 
5.49 
6.41 
8.24 
9.00 
9.61 













CAPILLARY MOVEMENT OF SOIL MOISTUEE. 



39 



Table 28. — Difttrilmfion of mois- 
ture ill flume 96. 



from underground water sources by capillary action of the soil. It 
must be kept in mind, however, that in the case of the flumes evapo- 
ration and capillarity are acting at the same time. 

DISTRIBUTION OF MOISTURE. 

The distribution of moisture in the flumes inclined upward at an 
angle of 15° does not differ materially from the distribution in 
the vertical flumes. In Table 28 is 
given the distribution of moisture in 
flume 96 at various times. It will be 
noticed that in this table, as in that 
for the vertical flumes, there is rather 
a uniform constant quantity of mois- 
ture near the low^er end and then a 
gradually decreasing amount to- 
ward the top of the flume. The rates 
of decrease, however, are not com- 
parable as far as the figures in this 
table and those for the other flumes 
indicate. 

In the open flumes there are several factors which account for a 
lack of uniformity in the distribution of moisture other than the 
mere fact of elevation above the surface of the water. The rate of 
evaporation is different for different points of the flume due to dif- 
ferences in moisture content 6i the soil (18). The concentration at 
the surface of the soluble salts of the soil, which will be different at 
different points throughout the flume, would cause some difference in 
the moisture content due to lessening evaporation. 



Distance. 


Percentage of water. 










Top 5 
inches. 


Bottom 
5 inches. 


Average. 


Inches. 


Per cent. 


Per cent. 


Per cent. 


28 


28. 32 


29.66 


28.99 


40 


28.56 


27.89 


28.82 


52 


26.70 


26.26 


26.48 


64 


24.83 


24.87 


24.85 


76 


2.5.06 


24.20 


24.63 


88 


21.71 


21.96 


21.83 


94 


20.58 


20.95 


20.77 


100 


17.25 


17. 73 


17.^9 



Table 29. — N n vi d e r of 
flume and angle of in- 
clination. 



EFFECT OF GRAVITY ON THE MOVE- 
MENT OF SOIL MOISTURE BY CAPIL- 
LARITY. 

As stated in this report, the plan was to 
have capillarity act in the direction of grav- 
ity, in a direction opposed to gravity, and in 
a horizontal direction in wdiich gravity was 
eliminated as far as possible. To give an idea 
of the influence of gravity in the movement 
of soil moisture by capillarity there are given 
below data on a complete set of flumes containing the heavy Eiverside 
soil. While the other soils show considerable variation, these varia- 
tions are almost entirely in degree and it is not thought that the ad- 
dition of these data to this report would be of any material benefit. 
Table 29 gives a list of the flumes in the set under consideration 
and their angles relative to the horizontal. 



No. of 


Angle of incli- 


flume. 


nation. 


34 


30" downward. 


32 


15° downward. 


31 


Horizontal. 


39 


15° upward. 


42 


45° upward. 


43 


Vertical. 



40 



BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 



There Avas in this set an additional flume inclined downward at 
an angle of 45°, but the resnlts from that flume Avere so near like 
those of the flume inclined at an angle of 30° downward that the 
addition of the data from this flume would be confusing without 
adding to the value of the information. In fact, the flume inclined 
downward at an angle of 45° was discarded after the third set 
of experiments, for the reason that it did not add to the information 
obtained from the flume inclined downward at 30°. 

Figure G gives the results of the daih^ measurements of the move- 
ment of moisture in the several flumes. Table 30 gives the distance 
the moisture had moved at diiferent periods of time from 1 to 40 
days. 

Table 30. — Distance inoistiirc had moved at variolar times, in flinnes plaral at 

<)'-fferc7it angles. 



Days. 






Flume 






















34 


32 


31 


39 


42 


43 




Inches. 


Inches. 


Inches. 


Inches. 


Inches. 


Inches. 


1 


26.00 


22.05 


20.00 


16.35 


16. 75 


15.70 


3 


44.15 


41.30 


33.75 


24.55 


24. 40 


20.75 


5 


58.25 


55.00 


41.20 


28.55 


28.85 


22.82 


10 


91.05 


8.85 


53.30 


35.80 


32.90 


26.25 


15 


118.65 


105. 20 


61.40 


40.25 


34.65 


28.05 


20 


144.05 


125.45 


66.15 


'i3.65 


36. 05 


29.40 


30 


181.25 


153.55 


75.80 


48.40 


37.50 


31.55 


40 




168. 35 


79.85 


51.25 


38.75 


33. 15 





Table 30 and figure shoAV very strikingly the effect of gravity on 
the capillary movement of soil moisture even at the end of the first 
day. It is obvious that in the horizontal flume the distance the 
moisture had moved is less than in either of the flumes inclined down- 
ward and is greater than for those inclined upward. This relation 
holds true not only for the first day but for all the time up to 40 
days. The table shows that the movement of moisture is less ex- 
tended in flumes inclined downward 15° than it is in flumes inclined 
downward 30°,- but that the difference is not nearly so marked as is 
the difference between the 30° flume and the horizontal one. Flumes 

31 and 32 show very clearly the effect of a relatively slight inclina- 
tion downward from the horizontal. 

For instance, on the thirtieth day the moisture has moved in flume 

32 a little more than twice as far as in flume 31. The figures pre- 
sented above and the figures obtained for the flumes inclined down- 
ward at an angle of 45° indicate that at least after an angle of 15° 
is obtained the effect of inclination is not nearly so marked, degree by 
degree, as for the first 15° of inclination. Comparing the horizontal 
flume with the flume inclined upward, we find that even on the first 
day the inclination is a marked factor in the extent of the movement 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



41 



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Fig. (!. — Rate of movement of moistue In set of flumes at various slopes each containing 
Riverside heavy decomposed granite loam soil. Figures within circles indicate point at 
which that number of liters of water had been taken up. 



42 



BULLETIN 835, U. S. DEPAETMEXT OF AGRICULTUKE, 



of the soil moisture. At the end of the thirtietli day rve iind that the 
Ihime .inclined upward at 45° gives only one-half as extensive a 
movement of the soil moisture as the horizontal flume, and the liume 
inclined upward at an angle of 30° gives about two-thirds as exten- 
sive a movement of the soil moisture. Taking the four flumes, the 
horizontal, the one inclined upward at 30°, the one inclined upward 
at 45°, and the vertical flume, the extent of soil moisture in distance 
wnthin these flumes is in the order given, with the greater extent of 
moisture in flume 31 or the horizontal flume. 

To show more clearly the effect of gravity upon the movement of 
moisture by capillarity. Table 31 gives the data of Table 30 in per- 
centa2:es of movement in the flume inclined downward 30° : 



Table 31. 



-Relative movement of moisture in flumes, expre^meiJ in pcrcentaoes 
of movement in' flume S.'f. 









Flume. 






Number 
of days. 
























31 


32 


31 39 

1 


42 


43 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


1 


100 


85 


80 


63 


64 


60 


3 


100 


93 


76 


56 


55 


47 


5 


100 


94 


71 


49 


49 


39 


10 


100 


90 


59 


39 


36 


29 


1.5 


100 


90 


52 


33 


29 


24 


20 


100 


87 


id 


30 


25 


20 


30 


100 


85 


42 


26 


21 


12 



On the thirtieth day we And that the moisture in the vertical flume 
has moved but 12 per cent as far as in flume 34 and in flume 42 it has 
moved 21 per cent as far; in flume 39, 26 per cent as far; in flume 
32, 85 per cent as far; and in flume 31, 42 per cent as far. It is 
obvious tliat the above percentages are comparable to the angles of 
inclination relative to the horizontal. This table brings out even 
more strikingly the eifect of gravity in the movement of soil moisture 
by capillarity. 

Table 32 gives the relative distance the moisture hatl lllo^■ed in the 
several flumes for diffea:'ent j)eriods of time, based on the distance the 
miosture had moved in 30 days in the resj^ective flimies. 



T-VBLE 32. — Capilhinj movement of moisture at varioii.s timc^ 
the mx^vement in 30 days. 



in prrcentaf/e of 









Flume. 






Number 
of days. 


























34 


32 


31 


39 


42 


3-1 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Percent. 


1 


14 


14 


2u 


34 


45 


50 


5 


32 


36 


54 


59 


77 


72 


10 


50 


53 


70 


74 


88 


83 


15 


65 


68 


81 


83 


92 


90 


20 


79 


79 


87 


90 


96 


93 


30 


100 


100 


100 


100 


100 


100 



CAPILLAKY MOVEMEISTT OF SOIL MOISTURE. 



43 



The striking feature of Table 32 is the fact that as the flumes re- 
cede from the vertical the rate of movement day by day is more uni- 
form and more constant. In the flume iiiclmed downward at an 
angle of 30° the extent of movement of moisUire on the fifteenth day 
or one-half the time was G5 per cent of the total movement of the 
moisture in 30 days. In flume 32 this percentage was 68. In flume 
31 or the horizontal flume it was 81 j>er cent ; in flume 39 it was 83 
per e&nt; and in the flume with a vertical angle of 45° it was 92 per 
cent. 

To present the above data m a more condensed form, figure 7 has 
been prepared. 




Fig. 7. — Comparison of rate of movement of moisture in flumes of various slopes ; all 
uumes containing Riverside heavy decomposed granite loam. Also shows appearance 
of moisture curves from top to bottom of flumes, except Nos. 32 and 39. 

Figure T shows the relative positions of the moisture in the 
various flumes with reference to the surface of the water in the tanks 
at various times during the experiment. The lines on the drawing 
showing the direction of the flumes represent the longitudinal axes 
of the flumes along their center lines. The figures show the direc- 
tions and the paths through which the moisture from the tanks must 
travel along the center lines of the flumes. It is obvious that during 
the forepart of the experiment the lines joimng the points represent- 
ing the positions of the moisture on the different dates are very ir- 
regular. It shows that there is a tendency of the curve joining these 



44 BULLETIN" 835, U. S. DEPARTMENT OF AGRICULTURE. 

points to become more uniform in outline as the experiment con- 
tinues for longer periods of time. That is, the line joining the points 
representing the position of the moisture on the thirtieth day is 
more regular and uniform than is the line joining the jDoints for the 
position of the moisture on the first daj. The figure indicates that 
with the la2:)se of an extended period of time the line joining the 
points representing the extreme extent of moisture would be of a 
parabolic form. This curve would have a rather limited extent in 
the vertical direction upward, but the longitudinal extent and the 
extent downward from the vertical might be infinity. Even with 
evaporation a factor, these last two named distances are relatively 
very great as compared with the vertical elements. The drawing 
emphasizes and portrays more clearly than do the figures the im- 
portance" of gravit}^ in the movement of soil moisture by capillarity. 
These deductions are of importance from the economic point of view 
in that they show very clearl_y what may be the distrilnition of mois- 
ture within the soil of water applied upon sloping ground. It in- 
dicates, for instance, that the extent of distribution of moisture down 
a slope would be much greater than it would be up a slope. A com- 
parison of the data for these flumes indicates how great would be the 
loss of water in conveying channels through capillary action Avhere 
the conveying channels traverse ground having a transverse slope. 
These data would indicate that on the lower side of the channel cap- 
illary action would continue taking water from the channel in 
about the same quantity for an indefinite period of time, while on 
the upper side the loss of water through capillarity would be very 
much less in quantity and in extent of time through which it would 
act. These figures indicate further the importance of slope of the 
strata of alluvial soil, both in reference to conveying channels and 
impounding reservoirs. In other words, these data indicate that 
with any appreciable slope downward of the strata, capillary action 
continues indefinitely. 

WATER USED. 

In considering the quantity of Avater used by the several flumes 
from the vertical upward to the 45° downward from the horizontal, 
it is found that the inclination of the flume is a most potent factor 
in determining the quantity of water that will be removed from the 
tanks. The data for these flum.es indicate clearly the effect of gravity 
in the movement of water as soil moisture by capillary action. A 
difference in inclination may mean, and most frequently does mean, 
a difference between practically no movement of soil moisture and a 
moA'ement of an appreciable relatively constant quantity of water. 

The figures within the small circles in figure 6 give in liters the 
quantity of water removed from the tanks. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



45 



An examination of these data sho-\As that the flumes inclined up- 
ward from the horizontal use a relatively large quantity of water 
during the firet two or three days and that after that time a relatively 
small quantity of water. Xear the end of the 30-day period very 
little water is taken up by these flumes. With the flumes inclined 
downward from the horizontal a somewhat larger quantity of water is 
used dviring the first three or four days than thereafter. However, 
these flumes after about the fourth or fifth day use a rather constant 
uniform quantity of water for an indefinite period of time within the 
limits of these tests. Table 33 gives the total quantity of water in 
liters used by the several flumes for different periods of time and 
shows in a more condensed form the data presented in figure 6, and 
that on the thirtieth day a vertical flume had used but 15 liters of 

Tabt.k ,13. — Total qiiantltu of iratcr used at various twies, in liters. 









Flume. 






Number 
of days. 




























34 


32 


.31 


39 


42 


43 


1 


10.0 


9.5 


10.0 


7.5 


10.0 


7.0 


3 


20.0 


18.0 


Ki.O 


13.0 


14.0 


10. 5 


5 


29.0 


25.0 


19.0 


15.5 


IG.O 


11.1 


10 


48. 5 


42.0 


2(i.O 


19.0 


19.0 


12.4 


15 


67.5 


59.0 


31.5 


22.0 


21.5 


13.5 


20 


87.5 


78.5 


3(1. 5 


24.5 


23.75 


13.8 


30 


124.0 


112.0 


45.0 


27.5 


28.5 


15.0 


40 


1.50. 


140. 5 


53.0 


31. 25 


30.5 


1.5.8 



water, while a fl.ume inclined downward at an angle of 30° had used 
124 liters, or about eight and a third times as much. The table also 
shows that, w^ith the exception of flumes 39 and 42, the quantity of 
water used by each flume was in the order represented by the in- 
clination of the flume from the vertical downward. This table shows 
that for the flumes inclined downward at angles of 15° and 30° there 
was not such a great difi'erence in the total quantity of watei* used. 
In other words, it would appear that for the flume inclined down- 
ward at an angle of 15° the capacity of the wick to furnish moisture 
to the flume from the tank had been about reached. In the two flumes 
39 and 42, or those inclined up at an angle of 15° and 45°, respec- 
tively, we find not much dilference in the quantity of water used. 
Just why this condition does exist in this case, there are not sufficient 
data to indicate clearly. However, flume 42 contains a relatively 
higher per cent of moisture than does flume 39. This of itself is not 
quite sufficient to account for the difference. 

On the fortieth day flume 43 had removed from the tank the equiva- 
lent of 9.64 inches, and flume 34 had removed the equivalent of 91.58 
inches. These figures are striking in that they show what effect the 



46 



BULLETTX 835, U. S. DEPARTMEK'T OF AGRICULTURE. 



slope of tke ground has in assisting- capillarity to drav,- vrator from 
conveying channels and storage reservoirs. 

Table 34. — Quantifij of iratrr I'eiiiorcil (vom tlic fujik^t at rnriniis times, 
exppresseti in (^epili, on an aren eqvoT to eroi^s .■^ertioii of flume. 









Flu 


me. 






Number 
of dm-s. 




























34 


32 


31 


39 


42 


43 




^ncJics. 


Incites. 


Tnchea. 


Inches. 


Inches. 


Inches. 


1 


<k 11 


5.05 


6.11 


4. 58 


6.11 


4.28 


3 


12. 22 


10.99 


9. 76 


7.93 


8.54 


6.41 


o 


17. 72 


15. 26 


11. 59 


9-. 40- 


9.76 


6.77 


10 


29.61 


25. 64 


15.86 


11.. 59 


11. .59 


7. 56 


15 


4:1.20 


36. 02 


19.22 


13.42 


12.81 


8. .54 


20 


53.40 


47. 90 


22.28 


14.95 


14. .50 


9.K 


40 


91. 58 


85. 75 


32.33 


19.06 


IS. 63 


9.64 



Table 3o gives the nnmber of cubic centimeters of moisture re- 
quired to advance the moisture in the flumes an average distance of 
1 inch at different periods of time. One point worthy of note in this 
table is the fact that flume 43 used about the same quantity of water 
per inch throughout. It must be tept in mind that this flume was 
closed to evaporation: and that no water escaped from this tank that 
was not confined within the wetted soil area of the flume. The other 
flumes were all open to evaporation. The figures seem to indicate 
that as w^e recede from the vertical the quantity of water re(iuired 
per inch is less. However, these figures are so confused with the 
evaporation that they do not indicate the true facts as to the require- 
ment of the soil itself when placed at these different angles. The 
evaporation factor is confused, for the reason that the soil within the 
flnime contains relatively different percentages of moisture, which 
has an influence upon the quantity of evaporation. Furthermore, 
the wetted area of soil differs so greatly in the several flumes and 
hence that the area exposed to cAaporation is much different. 

Tabt.e .35. — ArcnKjc quantity of irater required to adrmice wetted area in ftmnes 

1 ineli. 









riurae. 






. Number 
of days. 


























34 


32 


31 


39 


42 


43 




c.c. 


c.c. 


c.c. 


C.C. 


c.c. 


c.c. 


1" 


385 


419 


.500 


406 


596 


446 


3 


447 


436 


474 


530 


573 


506 


5 


498 


455. 


477 


543 


578 


486 


10 


533 


513 


488 


531 


587 


482 


15 


569 


561 


501 


556 


629 


487' 


20 


608 


626 


562 


573 


663: 


471 


30^ 


eS'i 


73? 


61« 


548 


760 


476 


40 




843 


681 


605 


767 


476 


50^ 










686 










1 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 47 

EVALUATION OF EMPIRICAL CURVES. 

In order to determine whether an}' mathematical relation could be 
found between the curves representing the movement of moisture in 
the different soils, mathematical equations to fit ' these empirical 
curves were found for typical flumes. The curves representing the 
movement of moisture in flumes at various slopes containing River- 
side heavy decomposed granite loam were evaluated to ascertain 
whether the movement of moisture w^as a function of the angle of the 
slope. 

The problem of finding a mathematical equation to fit a given 
curve is a tedious one. Since many soil physicists are perhaps un- 
familiar with methods of procedure other than by the method of 
least squares, which is so laborious as to limit its application, tlic 
method which w^as used to derive these formulse is explained in 
detail for two of these, one of which is a simple case and the other 
much more complicated. The method used is that explained in 
Engineering Mathematics, C. P. Steinmetz, New York, 1917, pages 
209-274, to which reference is also made for an explanation of the 
properties of different curves. 

In the following description, the number of days on which the 
moisture position was observed is denoted by x and the position of 
advancing moisture measured in inches above the water surface is 
denoted by y. The corresponding values of x and y were tabulated 
and plotted as a curve. It is apparent that the curve in every 
instance must pass through the origin, for wdien a?=0, y=0, and 
the nature of the problem also suggests that the curve be in the form 
of a parabola. This was found to be true in the majority of cases, 
but, as will be seen in the formulie given on a subsequent page, the 
curve law in some instances changed within the range of the 
observations. 

Curves which are represented by y^=asci^ are parabolic or hyper- 
bolic curves passing through the origin. When n is positive, the 
curve is parabolic. When n is negative, the curve is hyperbolic. 

The logarithm of the equation y:=ax^ is log j^=log a-\-n log x^ 
which is a straight-line formula. If the curve resulting from the 
plotting of the logarithm of y against the logarithm of x is a straight 
line, the curve representing the data is a parabola or hyperbola. 

The equation for the exponential curve is y=as,^'', which usually 
occurs with negative exponent in the form _^:=«£""-^, which gives log 
y=zlog a — nx log e. Log y is a linear function of x and plotting log 
y against a?, or log x against ?/, gives a straight line. Thus plotting 
log y and log x and x and y against each other permits the form 
of curve to be recognized. If constant terms exist, the logarithmic 



48 



BlTLLETTlSr 835, IT. S. DEPARTME:NT OF AGRICULTURE. 



line is curved. By trj'ing different constants, the logarithmic line 
changes in curAature, so that such constants may be found which 
make the logarithmic line straight. 

Logarithmic cross-section paper niay be j)iirchased which has both 
coordinates divided in logarithmic scale and also semilogarithmic 
cross-section paper having one ordinate so divided. When evalua- 
tions of equations having constant terms are to be made, these papers 
are very couAenient, since the curves may be plotted without looking 
up the logarithms; but since the method described by Steinmetz 



35 



30 



25 







if) 

o 



20 



IS 



10 



0.2 



0.4- 



Log days (Log x) 

0.6 0.8 I.O 1.2 /.4 



30 35 



1.6 









n/ 








-^ 




^ 


.-^ 


JC^ 




\.°J 


f] 


X 




X 






fA 


tp^ 












y 


o/^ 










J 


A 


Y 












































1.5 

> 

o 

{/) 

X 

u 
c 

CP 

o 

-J 

/.3 



4-0 



A 2 



Via. S. — Method of developing formula for movciueut of moisture in flume 43. 



requires logaritlnns to be tabulated in order to calculate the con- 
stants, common cross-section paper Avill usually suffice. 

In figure 8 the data representing moisture movement in flume 
i3 are plotted. The values of log ?/ and log ,r are also plotted and 
found to be a sti'aiglit line, so that log //=log a-\-'n. log ,r and the 
curve is a parabola. Table 36 gives the data, the logarithms of 
a and ?/ and tlie calculated log y as obtained from the formula which 
was derived. 



CAPILT.ARY MOVEMENT OP SOIL MOISTURE. 49 

Table SQ.— Flume Ji3. 



X 

days. 




Logj; 


Logy 


1.224 


7/ * 




U 
inches. 


(log 
days). 


(log 
inches). 


+ .183 
log X. 


(inches). 


A 


1 


15.70 





1.196 


1.224 


16.75 


+ 1.05 


2 


18. 95 


.301 


1.278 


1.280 


19. 05 


+0.10 


3 


20.75 


.477 


1.317 


1.313 


20.56 


-0.19 


4 


22.00 


.002 


1.342 


1.336 


21.08 


-0.32 


5 


22. 82 


.099 


1.358 


1.354 


22.59 


-0.23 


G 


23. 75 


.778 


1.376 


1.369 


23.39 


-0.36 


7 


24.45 


.845 


1.388 


1.381 


24.04 


-0.41 


9 


25.25 


.954 


1.402 


1.402 


25. 24 


-0.01 


1» 


25. 75 


1.000 


1.411 


1.410 


25.70 


-0.05 


11 


26. 25 


1.041 


1.419 


1.419 


26. 25 


0.00 


V2 


26. 75 


1.079 


1.427 


1.425 


26. 61 


-0.14 


13 


27.15 


1.114 


1.434 


1.431 


20.98 


-0.17 


15 


27.75 


1.176 


1.443 


1.443 


27. 75 


0.00 


17 


28.37 


1.230 


1.453 


1.453 


28. 37 


0.00 


28 


31.00 


1.447 


1.491 


1.493 


31.12 


+0.12 


39 


33.00 


1.591 


1..519 


1.520 


33.11 


+0.11 


4S 


34.00 


1.081 


1.531 


1.536 


34.36 


+0.36 


50 


34.75 


1.699 


1..541 


1.540 


34. C8 


-0.07 



* j/c. distance in inches computed by using the formula derived for flume 43. 

The 18 sets of observations are divided into two groups of 9 each. 
Tlie Slim of the first 9 log x and log y are found, togeth.er with the 
second group of 9. These are indicated as 29 in the computations. 
Since the' formula log y= log a -\- n log x applies to all parts of the 
curve, it is the same for the two groups, subtracting the two groups 
from each other eliminates log a and dividing the one difference A by 
the other gives the exponent n^ 

log 2/2 — log 2/1 
log X., — log c»i 

The sum of all the values of log .r, S^g, is found and multiplied by 
iu and the product subtracted from the sum of all the log y, log « = 
log y—n log X. The difference. A, is divided by 18 and the quotient 
is the log a. 

The actual computations for the above case are as follows : 



loiT 



r= 12.060 



log // 



= 12.068 
= 13.259 



A = 



n. = 



6.403 
1.191 
6.403 
^ -17.716 
17.716 X 0.186 = 



A= 1.191 



0.186 



log X 



log yS,3 = 25.327 
3.295 



A = 

22.032 ^ 18 = 1.224 = log a 
a — 16.75 
?/ = 16.75.i^o-i«« 

147697"— 20— Bull. 835 i 



22.032 



50 BULLETIISr 835, U. S. DEPARTMENT OF AGRICULTURE. 

Table 37. — Flume 31. 























^■c*= 




(days) 


y 

(inches) 


loga; 
(log 
days) 


logy 

(log 

inches) 


J'. 


v~ y\ 
- y 


■Ics y; 


Vi +5.5 


leg (.i/2 

+ 5.5) 


9.r31 
+1.06 
Irgx 


:(0. 43 

2-l.OS 

-5.5) 


A 


1 
2 
3 

4 
5 
fi 
7 
8 
9 
10 
11 
12 
13 


20.00 
28. 45 
33.75 
37.90 
41.20 
44.60 
47.00 
49.15 
51.30 
53.30 
55. r.o 
57.50 
59.05 



.301 

.477 

. 602 

.699 

.778 

.845 

.903 

.954 

1.000 

1.041 

1.079 

1.114 


1.301 

1.4.54 
1.528 
1.579 
1.615 
1.649 
1.672 
1.692 
1.710 
1.727 
1.748 
1.750 
1.771 


20.00 
28.21 
33.96 
37.54 
40.71 
43.80 
46.59 
49.14 
51.51 
53.73 
55.-81 
57.79 
59.67 




-.24 

.21 

-.66 

.51 

-.40 

-.41 

-.01 

.21 

.43 

.21 

.29 

.62 



9.380 
9. 322 
9.820 
9.708 
9. 602 
9.613 

9.322 
-9.633 
9.322 
9.462 
9.792 












i ■""" " I 1 




! 1 i 




1 1 




1 1 




i 












1 






1 1 




























6.12 


.793 


.812 


58.68 


-0.37 


14 


60. 03 


1.146 


1.778 


61.47 


1.44 


.1.58 


6.94 


.-841 


.846 


59.96 


-0.07 


15 


61.40 


1.176 


1.7-88 


63.19 


1.79 


.2.53 


7.29 


.863 


.878 


61.14 


-0.20 


ir. 


62. 35 


1.204 


1.794 


64.84 


2.49 


.396 


7.99 


.903 


.907 


62.37 


+ 0.02 


17 


63.25 


1.230 


1.801 


66.43 


3.18 


5.07 


8.68 


.939 


.935 


63.32 


+ 0.07 


18 


64.15 


1.255 


1.807 


67.97 


3.82 


.582 


9.32 


.969 


.962 


64.31 


+ 0.16 


19 


65. 20 


1.279 


1.814 


69.46 


4.26 


.629 


9.76 


.989 


.987 


65.26 


+0.05 


20 


66. 15 


1.301 


1.821 


70.89 


4.74 


.675 


10.24 


1.010 


1.010 


66. 15 


O.CO 


21 


67. 10 


1.322 


1.827 


72.29 


5.19 


.715 


10. 69 


1.029 


1.033 


67.00 


-0.10 


22 


67.90 


1.342 


1.832 


73.65 


5.75 


.760 


11.25 


1.053 


l.O.K 


67. 85 


-0.05 


24 


69.35 


1.380 


1.841 


70.26 


6.91 


.840 


12.41 


1.094 


1.094 


69. 34 


-0.01 


25 


70.30 


1.398 


1.847 


77.51 


7.21 


.-858 


12.71 


1. 104 


1.114 


70.01 


-0.29 . 


27 


71.45 


1.431 


1.854 


79.94 


8.49 


.929 


13.99 


1.146 


1.148 


71.38 


-0.07 


29 


72.60 


1.462 


1.861 


82.25 


9.65 


..985 


15.15 


1.180 


1.181 


72. 58 


-0.02 


30 


73.02 


1.477 


1.863 


83.38 


10.36 


1.015 


15. 86 


1.200 


1.196 


73. 18 


+ 0.16 


32 


74.35 


1.505 


1.871 


85. 56 


11.21 


1.050 


16. 71 


1.223 


1.226 


74.23 


-0.12 


35 


■75. SO 


1.544 


1.880 


88.68 


12. SS 


1.110 


18. 38 


1.264 


1.267 


75. 69 


-O.U 


38 


77.25 


1.580 


1.888 


-91.64 


14.39 


1.158 


19. 89 


1.299 


1.306 


76.91 


-0.34 


■53 


78.90 


1.633 


1.897 


96.29 


17.39 


1.240 


22. .S9 


l.SfO 


1.363 


78.72 


-0.18 


4f. 


79.95 


1.663 


1.903 


98. 93 


18. 98 


1.278 


24.48 


1.389 


1.394 


79.66 


-0.29 


49 


80. 60 


1.690 


1.906 


101.45 


20.85 


1.319 


26. 35 


1.421 


1.423 


80.48 


-0.12 


55 


81.90 


1.740 


1.913 


106.25 


24.35 


1.387 


29.-85 


1.475 


1.475 


■81.90 


0.03 



* ijc, distances in inches computed by using the formula derived for f.uine 31. 

In tabk 37 are given the data obtained from flume 31. the 
log-aritlims of x and y being tabulated. Figure 9 shows that the 
logarithmic curves between log ,r, log ?/, and .r and y are not straight, 
so that the curve is not a simple parabola or exponential curve. The 
curve between log .r and log y is straight up to 1'2 days. Thus tlie 
curve is a simple parabola for values of x less than 12. The 12 sets 
of observations are divided into two groups of six each and the 
formula derived as explained for flume No. 42. 

loo- «• S,=2.857 



2, =5.823 

A =2.966 

.,_1.182 



2.966 
log X ^12 =8.680 
8.680 xu = 



=0.40 



log yjl^= 9.126 
2,= 10.308 
A= 1.182 



log //,2,, =19.435 

3.472 

A =15^62 



15.962-^12 = 1.330=log a 
log ?y 1=1. 330 +0.40 log x 
2/1=21.39 a-o" 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



51 



The values of y^ are calculated from this formula and tabulated in 
Table 37. 

The ditferences bet\Yeen ?/, and y are also tabulated as y... .?/.,= 



O ,1 .2 



I 1.2 1.3 l.a I.S 1.0 1.7 1.6 .. 




Days (X) 
Fig- 9. — Method of developing formulse for movement of raoibluro in iiuiuo ".1. 

The values of y.^. log y.., log t/^ x, log .r, and y arc plotted against 
each other as shown in figure 9 ^i, but none of these curves is a 
straight line. This suggests the existence of a constant term, and a 
number of constants were tried until it was found that the curve be- 
tween log (2/0+6.5) and log x is a straight line. The curve above 12 
days or 12a; then is log (?/.+5.5)=log a-\-n log x. 



52 BULLETIN 835, U. S. DEPARTMENT OF AGEICULTURE. 

The remaining 22 sets of observations were divided into two groups, 
and the equation of tliis parabola was dr^ived as follows: 

log X 2,,=13.751 log (y2+5.5)S,,=10.481 

S,,r:rl7.124 S,,r^l4.061 

A := 3.374 A = 3.579 

;-_ 3.579_ log (y,+5.5) i:,3:=24.542 



3.374 30.875 Xl.06=32.r.r>r, 

log X S,,=30.875 A = 1.876-10 

1.876— 10-^22=0.031— 10=]og a 
log (?/,+5.5) =0.631 — 10+1.06 log x 
^2=0.43 ,ci-p«— 5.5 
Since y._ = rj,—y, y=y^—y^ 
then ?/=21.39 .r^'*— (0.43.ri'"^— 5.5) 

The values calculated from this equation are tabulated and the 
differences from the values of y as obtained in the experiment are 
noted. 

When the curve resulting from the plotting x or y against log x 
or log y is straight, the exponential curve is derived in the same man- 
ner as for a parabola. The data are divided into two groups and the 
value of 11 and log a found. 

log y=:log a — )ix log £ represents the equation for both groups, so 
that log a can be eliminated by subtracting one from the other. 

n=, logjA-log_^5 ^ in which log £=0.4343 

log£ (loga',— log;rJ 

log «=:log // — nx log £ 

In several eases it was found that for high values of x and y the 
curves were straight lines and the equations for these straight lines 
found. 

Subtracting the values of ?/^ in the equation y^-=imx-\-h from the y 
values of the data gave values of y^. 

The log y., plotted against x gave straight lines, so that the curve 
for these low values of x and y were exponential curves which were 
derived as explained above. 

The formula} for the curves representing moisture movement in 
the flumes held at different angles when filled with Riverside heavy 
decomposed granite loam (Placentia loam) were as follows: 
Flume Xo. 42 (45° up) : 

7/=33.7+0.l2^'— (Ib.oe-^'--') 



CAPILLARY MOVIiMENT OF SOIL MOISTURE. 53 

Flume No. 32 (15° down) : 

?/=21.44.z.'°-^«— (0.(>26a;i-9-— 11) 
Flume Xo. 34 (30° down) : 

?/i=22.24a;''-«^ 
Flnme Xo. 39 (15° up) : 

y=18.3Cv(,'0--« 
Flume No. 31 (horizontal) : 

7j=2i:Sd.>"*— (0.43.y/-o«— 5.5) 
Flume No. 43 — (vertical up) : 

//=16.75,«'^-i» 
The following eciuations were found for other flumes and soils: 
Flume No. 33 (15° down) Riverside heavy decomposed granite 
loam (Placentia loam) Riverside, Calif. 
^=:5.1,i'+21.— (18.25£-"-«=^) 
Flume No. 61 (45° up) Dublin clay loam, near Whittier, Calif.: 

7/=0.21,i'4-23.T— (15.5e-""^') 
Flume No. 51 (horizontal) Dublin clay loam, near Whittier, 
Calif.: 

2/=11.23^'''" 
Flume No. 59 (15° up) Dul)lin clay loam, near AYliittier, C\dif. : 

?/=15.21.K«-" 
Flume No. 40 (15° up) Riverside heavy decomposed granite 
loam (Placentia loam) Riverside, Calif.: 

yz:r20.53*'°-3l 

Flume No. 30 (horizontal) Riverside heavy decomposed granite 

loam (Placentia loam) Riverside, Calif.: 
yr=20.89a?°-*^ 
Flume No. 35 (30° down) Riverside heavy decomposed granite 

loam (Placentia loam), Riverside, Calif, (for values of os 

greater than 8, curve is straight line) : 

^r=7.3a' + 12 

These equations could be used to determine the position of the 
moisture at some time beyond the range of observation of the experi- 
ment if it is assumed that the curve law does not change for higher 
values of x. 

Dr. R. H. Loughridge, in the Report of the College of Agriculture 
of the University of California for the years 1892, 1893, and part of 
1894, pages 91 to 100, gives the observed position of moisture in a 
column of Ventura County "tilled soil (silt loam). These observa- 
tions extended for a period of 195 days, w^hich is one of the longest 
periods that has been reported in literature. The formula //=:13.9.2;"-* 
represents the movement of this moisture and there is no change in 
the curve throughout the period of observation. Values of y calcu- 



64 BULLETIIT 835, U. S. DEPAETMENT OF AGRICULTURE. 

luted fi'om this formula agree with sufficient accuracy with the ob- 
served values of //. 

Dr. Loughridge states that the limit of moisture movement was 
reached at the end of 195 days at 50 inches. It is interesting to note 
that the position of the moisture at the end of one year as calculated 
from the formula would be 56.2 inches ; at 390 days, twice the time of 
observation, 57 inches: tAvo years at G6.2 inches; and three years, 72.9 
inches, or only 22 inches above what it was at the end of 195 days. 

OPEN VERSUS COVERED FLUMES. 

The results obtained from the covered flumes are very similar to 
those ol)tained from the flumes open on top to evaporation. With one 
or two exceptions the results with the covered flumes do not differ 
materially from what could have been foreseen from the results with 
the open flumes. The essential ditference is one of degree, as would 
have been expected. One striking exception is the fact that in every 
instance of the 25 or 30 experiments the open flume has the more 
rapid rate of movement of the moisture for the first one to five weeks 
of the experiment, the difference in time depending upon the char- 
acter of the soil. The heavier the soil and the longer the open flume 
maintained the more rapid rate of movement of the moisture. The 
more rapid rate of movement is maintained irrespective of evapora- 
tion. This fact will be more clearly seen from the data submitted 
below. There is, as would be expected, a small difference in the rela- 
tive percentages of moisture contained in two flumes, and especially 
is this difference noticeable in the u^^per layers of soil. 

Inasmuch as the results with the covered flumes differ only in de- 
gree from those of the open flumes, it is not deemed that the sub- 
mission of all the data and its discussion would add materially to the 
value of this report. For that reason there will be discussed only 
(ine covered flume in its relation to its comparable open flume. The 
two flumes that will be presented in detail are the horizontal flumes 
70 and 71 containing the soil from Upland. This is a gravel and 
sand soil containing but little clay. The selection of this particu- 
lar soil for presentation is merely for convenience, as the results 
obtained by its use are similar to the results obtained from other soils, 
figure 3 (p. 23) shows the curves representing the movement of 
moisture in these two, flumes. 

Table 38 gives the total movement of moisture in these two flumes 
at the end of various periods of time. From this table it will be 
observed that flume 70, which is open to evaporation, has the more 
rapid rate of movement of the moisture up until the fifth day. After 
the fifth day flume 71. or the covered one, has a more extended move- 
ment of the moisture and upon the thirtieth day this difference is 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



55 



Table 38. — Movement of 
moisture at various times, 
in inches. 



about 9 per cent in favor of the open flume. The rate of movement 
of the moisture in the closed Hume is more uniform tliroughout the 
30 clays than that in the open flume. The facts just stated would 
appear to be contrary' to what might have been forecast, for the 
reason that evaporation from the open flume would deprive that 
flume of some of the water .furnished by the wick. In the closed 
flume practically all of the water furnished by the wick would be 
available for the capillary action of the 
soil. These results would indicate first 
that in the closed flume the soil in the 
flume proper could not use all of the w^ater 
that the wick was capable of furnishing. 
This would indicate a friction factor 
caused either from partially confined air 
or otherwise that would not appear to 
occur in the open flume. It is found in 
the open flume that either from evaporation 
or from a more ready circulation of the 
air the capillary action of the soil within 
the flume was stimulated or that the fric- 
tion was reduced. From observations made in connection with other 
experiments it seems to the writer that the fact of more rapid 
rate of movement in the open- flume at the beginning of the experi- 
ment is due to both of these factors. It is known that " trapped *' air 

has an effect upon capillary ac- 
tion and that evaporation would 
stimulate the circulation of the 



Number 
of days. 


Flume. 


70 


71 


1 
3 
5 
10 
1.5 
20 
30 


Inches. 
23.10 


/«cftes. 

21.30 


■U.70 
.54.60 
fi4.00 
70.15 

80.05 


41.30 
54. 80 
f)o. 50 
73.70 

87.10 



Table 39. — Quantity of water used at 
various times, in liters, and in per- 
centages of total used in SO days. 







Flume. 




Number 
of davs. 


















70 


71 


70 


71 




Liters. 


Liters. 


Per cen'. 


Per cent. 


1 


6 


C 


IS 


21 


3 

5 










12 


12 


37 


42 


10 


17 


17 


51 


59 


15 


21 


21 


64 


72 


20 


26 


24 


79 


.«3 


30 


33 


2" 


100 


100 



air. 



Table 39 shows that a rela- 
tively greater quantity of water 
was used by the closed flume dur- 
ing the forepart of the experi- 
ment than was used by the open 
flume. This is a condition which 
would be anticipated, as evapo- 
ration deprives the open flume of 
part of the water furnished by 
the wick. The table shows very clearly that the covered flume does 
not tax the wick to its capacity in furnishing water from the tank to 
the flume proper. 

Table 40 gives the quantity of water required to move moisture 
in the flume an average distance of 1 inch for different periods 
of time. This table does not show effects other than would have 
been anticipated. It is observed that there is a greater use of 
water on the thirtieth day in flume 71 than during the fore part 



56 



BULLETIN- 835, U. S. DEPARTMENT OF AGRICULTURE. 



of the ex23eriment. Tliis can be accounted for in two ways: First, 
all evaporation could not be eliminated without liability of trapping 
the air within the flume. Second, there is, as has been shown 
previously, an increase in the percentage of moisture contained in 
different portions of the flume with the age of the experiment. 

Table 41 gives the use of water by these flumes in equivalent depth 
over an area equal to the cross section of the flumes. 



Table 40. — Water required 
(it ra/'/oM.s tiinefi to ad- 
rance r.ioisturc an aver- 
age distance of 1 inch. 





T":um:>. 


Numlior 
of days. 












70 


71 




c. c. 


c. c. 


1 


2o'J 


2S1 


5 


28S 


291 


10 


311 


310 


13 


328 


321 


20 


371 


326 


30 


412 


333 



Table 41. — ^Yatcr removed 
from tanks at various 
times, in depth. 



Xumlicr 
of day.^-. 


Fiumo. 


70 




1 
5 
10 
15 

20 
30 


Inches. 
3. 66 
7.32 
in. 37 
12.81 
l.j. 86 
2i!. 13 


l.'rh(s. 
3.66 
7.32 
10.37 
12.81 
1 1. 64 
17.6:) 



It is found that flume 70 used the. equivalent of 20.13 inches of 
water in 30 days, while the covered flume (71) used the equivalent of 
17.69 inches or about 12f per cent less than the open flume. These 
flgures show that for the last ten days of tiie experiment the open 
flume used 4.27 inches and the closed flume 3.05 inches or a little over 
25 per cent less water than the open flume. These last figures would 
represent the effect of evaporation. In other Avords, during the last 
ten days of the experiment evaporation from the flume took care of 
at least 25 per cent of the w^ ater furnished by the wick. 

EFFECT OF TEMPERATURE ON SOIL-MOISTURE CONDITIONS. 



As has been stated previously, a temperature at and below the 
freezing point appears to have influenced to a marked extent the dis- 
tribution of moisture within the flumes. Some few soil samples taken 
from the flumes during the winter of 1916-17 gave results contrary 
to what was to be expected. In the sampling of the flumes, two 
samples were taken from each point of sample. The soil from the 
top o inches was placed in one bottle and the soil from the bottom 
5 inches in a second bottle and the moisture determined for each 
separately. There are two basic reasons wdiy the percentage of mois- 
ture in the top samples should be less than that in the samples from 
tiie bottom 5 inches. First, the sample from the upper 5 inches 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



57 



of soil is farther away from the water and gravity wouUl tend to 
hold the moisture in the lower layer. Secondly, evaporation from the 
surface would tend to further reduce the moisture at and near the 
surface. Thus the laws of physics would indicate a lower percentage 
of moisture toward the top of the flume than near the bottom. There 
were, however, several instanced where this relationship was inter- 
olianged, and more especially was this noticeable during the winter of 
1916-17. When this interchanged relationship in the distribution of 
moisture was observed so frequently during the spring of 1917 as to 
almost preclude the probability of error from sampling, it seems 
evident that the unlooked-for distribution of moisture was the result 
of some natural condition. It soon became apparent that the top part 
of the flumes showed the greater percentage of moisture during only 
that time of the year when the air temperature was or recently had 
been below 30°. In looking back over the results of the preceding 
winter, this same condition was found. When these facts became 
evident it Avas so late in the season that there w^as no opportunity to 
prove the matter beyond a question of doubt. For this reason a few 
of the samples, with percentage of moisture and air temperature, are 
given in Table 42 for what they may be worth. 

Tahle 42. — Soil-moist tti'c ili-strihution ami nir teinperdturc. 



Date. 


Distance. 


Flume. 


Percentage of 
maisture. 


Temperature for 
week preceding. 


Top Bottom 
5 inches. ; 5 inches. 


Maxi- 
mum. 


Mini- 
mum. 


Mar. 5, 1917... 

Mar. 16,1917.. 
Apr. 21, 1917.. 


Inches. 

21 

38 

62 

92 

44 

128 

201 

32 

68 

104 

140 

190 

32 

72 

56 


90 


Per cent. 
31.53 
27.44 
26.33 
2:5.37 
29.11 
27.78 
41.13 


Per cent. 
29.32 
36.30 
25.66 
23.67 
26.85 
26.64 
28.46 


70° 27° 




1 




1 


95 


70° 


27° 








92 


28. 75 30. 44 
26. 45 26. 38 
25. 75 25. 45 

24.30 ! 24.60 
17. i<3 '• 19. 32 

28.31 1 29.90 
20. 61 ! 22. 00 
25.13 25.47 


77° 


26° 














101 


82° 32° 















At a distance of 82 inches in flume 93 there was taken on ]March 
20 a set of samples dividing the boring into four samples, each 
containing 2-| inches of soil in depth, and the following results ob- 
tained : 

In the top sample, 28.96 per cent. 
In the second sample, 27.56 per cent. 
In the bottom sample, 26.60 per cent. 
In addition to the samples given above there are several others 
showing similar results. There are some samples taken at the same 



58 BULLETIN 835, IT. S. DEPARTMENT OF AGRICULTURE. 

time in the same fiume that gave tlie natural distribution of moisture 
and the interclianged distribution. In these cases there v;as not 
as great a difference in the relative percentages of moisture at the 
top and bottom as where all samples showed the interchanged re- 
lation. 

In the samples given above, it is noticed that this interchanged 
relation of the distribution of the moisture occurs in both the open 
and covered flumes. This same fact is true of all of the other work, 
except that the covered flumes seem to require a little lower tem- 
perature of the air to cause this result than do the open flumes. It 
will be noticed in Table 42 that with a relatively low percentage 
of moisture an interchange of the natural distribution of tlie mois- 
ture did not occur. It is probable that if such a distribution should 
occur, a temperature low^er than 26° F. would be required. As shown 
in this table, for flume 101, with the minimum temperature of 32", 
the upper part of the soil still contained a little less moisture than 
the bottom part of the soil. By comparing results shown for flume 
101 with other samples taken with higher minimum temperatures, 
it is evident that a slight difference occurred in the normal distribu- 
tion of the moisture in the samples. 

Before a definite conclusion can be drawn, additional experiments 
will have to be made. 

THE CAPILLARY SIPHON. 

The definitions of capillarity and of capillary moisture used in 
so nuiny of the old textbooks would lead one to conclude that free 
water would not be developed as a result of capillarity. For in- 
stance, the old illustration of the towel and the basin of water was 
used to combat the idea of free water as a result of capillarity. No 
reference to the probable fallacy of the old doctrine has been stated. 
In fact, all reference to the relation of gravity and capillary action, 
except as contained in the old original definition, has been in the 
most general terms. The prevalent method of disposing of the ques- 
tion is to say that capillary action is influenced by graYitj. (1) 
There appears to be no statement as to any quantitative relation. 

One of the very first sets of experiments tried at Riverside in the 
fall of 1915 included flumes inclined at angles of 15° and 30°, and one 
at 45°. The first of these had an ultimate total length of 20 feet 
and the last two had lengths of 10 feet each. The moisture in the 
flume inclined downward at 45° had reached the end of the flume 
in 18 days, and in the one inclined downward at 30°, the moisture 
had reached the end of the flume in 21 days. Three or four days 
after the moisture had reached the end of these flumes, free water 
was observed dripping from the ends of both. In about a week 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 



59 



after tlie moisture had readied the loAver end of the ikiiiie hiclined 
downward at an angle of 15° free water commenced dripping from 
the lower end. The water continued to dri]) from the ends of all 
three of these flumes for at least two weeks, or until the flumes were 
dismantled. It must be kept in mind that this water Avas raised 
fix)m the tank a vertical distance of 4 inches by capillarity and 
against gravity. It was then transmitted doAvn the flumes by means 
of the same force and in a direction with gravity. The moisture 
left the soil column at the lower end of the flume as free water, drop- 
ping to the ground. At no point in the entire lenglh of the soil 
column, with the possible exception of the extreme lower end of the 
flume, was the percentage of moisture in the soil as great as that of 
capillary saturation, as measured by tlie 
general methods for determining tliLs 
percentage. This, then, is in effect 
transferring water from a bod}^ of free 
Avater by capillarity and delivering it 
again as free water. 

To supplement the results from the 
flmnes and to test the further possibility 
of creating a capillary siphon, a sj)ecial 
piece of apparatus shown in figure 10 
was set up. 

A-B in figure 10 is a galvanized-iron 
tube 7 by T inches in area and made in 
the shape shown. This box is water- 
tight and air-tight, except along the top 
X-B, at the bottom of the short arm at 
C, and at a point B at the bottom of the 
long arm. This tube stands vertical 
and rests on A. The top along the line 
X-B is open to the air. The lower end 
of the short arm at C has soldered over it 
a fine-meshed wire gauze. /> is a f-inch ell soldered into the lower 
end of the long arm; the top of the ell is fitted with a water-gauge 
connection. Into the top of this ell is flatted a gauge glass X-Z>, on the 
outside of the tank or tube. The tube is packed with soil as indicated 
and the soil is exposed to the air along the line X-B. The shoit 
arm of the tube extends down into a tank of water represented by 
water line in tank. 

It is observed from figure 10 that the high-water line in the 
tank is 8 inches below the bottom of the horizontal part of the tube. 
This 8 inches is then the distance the water must be raised from the 
tank before it can move horizontally. It mnst then move hori- 




FiG. 10. — The soil coluiim as a 
capillary siphon. 



60 BULLETIN 835, IT. S. DEPARTMENT OF AGRICULTURE. 

zontally an average distance of 12 inches before it am move 
do\rnward. 

Tiie detailed measurements will not be given, but after CO days the 
water in the gauge glass on the outside of the flume showed water 
up to a point within 11 inches of the surface of the water in the tank; 
that is, after 60 days that part of the tube below the point desig- 
nated " Gage E "' in gauge glass was completeh' saturated. After 
the sixtieth day, the rate at which the water rose in the gauge 
glass was very slow, and upon the seventieth the experiment was 
terminated. 

This experiment, as did the previous ones cited, gave free water 
as a result of capillary action. 

Three additional experiments were run with the same tube, but 
containing soils of a different type. In each case the same result was 
obtained, except that they Avere terminated sooner and for that reason 
the water did not rise so high in the glass. 

Finally, it uiry be stated that in every flume, covered and open, 
that was inclined downward at an angle from 15° to ^5° free water 
was developed when the experiment was run for a sufficient time. 
In only 3 or 1 instances out of the 20 or more flumes so inclined 
were the experiments terminated before free water was dripping 
from the lower ends of the flumes. 

Several tests were made of the amount of water taken up from 
the tanks and delivered again at the lower end of the flumes as free 
water. One of these tests will be given. 

The flume selected is No. 95, containing the lava soil from Idaho. 
This flume was covered, inclined downward from the horizontal at 
an angle of 30°, and was 15 feet in length. The records show that 
the flume commenced dripping water at the lower end on Feln-uary 
25, 1917. Commencing with March 1, the quantity of water lost 
from the tank by the wick was 18 liters. During this same period 
there was caught at the lower end of the flume 8.78 liters, or approxi- 
mately 50 per cent of the quantity taken from the tank. The water 
was caught in a can as it dripped from the flume. 

It has been yuggested that a true siphon might have been formed 
as a result of " soil puddling " or other natural mechanical means. 
It did not occur in man}^ cases and it is doubtful if it occurred at all. 
It is found, for instance, that with the use of clean, coarse build- 
ing sand, devoid of clay or other fine material, the same result is 
obtained. However, to test this point further, a system of ventila- 
tion within the wick was installed. 

Ventilating pads were made out of ordinary window-screen wire. 
From six to eight thicknesses of wire were rolled into a very small 
diameter and then flattened out. This made a pad of wire about 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 61 

2^ inches in width and about three-eighths of an inch in thickness. 
The wire, when pLiced within the soih kept the soil particles apart 
thronghout most of the spaces occupied by the pad. Four of these 
wire pads were inserted vertically within the wick, extending from 
within about one-half inch of the water in the tank up through the 
wick of the flume to the air above. These pads were placed in the 
corners of the wick and about 1 inch from any side. The flume and 
wick were' then packed with soil and the experiments started. With 
the flume inclined downward at an angle of 30°, and with the light 
sandy Idaho soil, water dripped froui the end of the flumes in about 
four days and continued to drip until the experiment was discon- 
tinued. This experiment was repeated, and in addition to the verti- 
cal ventilating pads, two other pads were placed, one diagonally 
across the wick and one in a horizontal position. The ends of these 
pads butted against the vertical pads and were placed about 1 
inch above the surface of the water of the tank. 

This flume gave the same results as the other flume, but a little less 
water Avas taken from the tank in the case of the ventilated wicks 
than in the wicks not ventilated. However, free water dripped from 
the lower end of all of these flumes. In the Avick having the vertical 
and horizontal pad ventilators (so called) there was no unventilated 
space Avithin the Avick at a greater distance than 1^ inches from a 
ventilator. 

In several of the flumes inclined doAvuAvard, Aarious other means of 
ventilating the wick were tried and in each case free Avater Avas still 
given off at the lower end of the flume. 

A flume inclined downAvard at an angle of 15° and 20 feet long Avas 
filled Avith clear Santa Ana River sand. This sand contained practi- 
cally no fine material and only traces of organic matter. Yet this 
flume, like the others described aboA-e, gave free aa ater at the loAver 
end of the flume, and within a Aveek from the time the experiment 
AA'as started. 

It AA'ould seem, therefore, from the evidence of the ventilated Avicks 
and flumes filled with types of soil from A^ery coarse sand to fine 
clay and all giving off free water, that the capillary siphon, as above 
styled, is perfectly established. 

It would also seem that capillary siphons occurring in nature might 
not be uncommon and that such siphons, first by capillarity alone, 
and later assisted by gravity, might cause the swamping of lands. 
Such a condition might arise if there were a stratum of soil of rather 
high capillary power and a rather impervious subsoil; if the upper 
end of such a soil arrangement were in contact with a body of water 
and the Avater did not have to be lifted too far by capillarity, and 
from that point the soil and subsoil had a slope downward at an angle 



62 BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 

at least as great as 15°, then it would lla^•e the condition of the 
flumes above described. If, now, there were a sudden change in the 
slope of the ground toward the horizontal, or if the more loamy soil 
verged into a denser soil, free water might be developed at this point 
as the result of capillary action. 

The capillary siphon might develop, also, in an earthen reservoir 
dam with a puddle or concrete core wall extending only to the flow- 
line or slightly above it, and under certain conditions produce satura- 
tion in the lower side of the dam. 

That a capillary siphon as above described is in accord with 
physical laws and was not the result of mechanical defects or error 
in manipulation is readily proven. Briggs (13) and Widtsoe and 
McLaughlin (19) have shown that the quantity of .water retained by 
a soil column against gravity depends upon its length. Also that 
a column 1 foot in length Avill hold at all points a greater percentage 
of water than a column 2 feet in length. Hence, as the length of the 
inclined flume is greater, the percentage of moisture held against 
gravity will be smaller. It would follow-, therefore, that beyond a 
certain length of the inclined part of the flume, not all of the water 
furnished by the wick could be retained against gravity by the in- 
clined part of the flume. 

It has been shown in this report that the distribution of moisture 
in vertical soil columns does not decrease uniformly with height 
above water. It has been indicated also that the greatest percentage 
of moisture in the vertical column may not be at the immediate water 
surface. From moisture analyses made of samples from vertical 
fiumes, noted in this report, and from a great many other special 
experiments, the writer will sa}^ that the greatest percentage of mois- 
ture in a vertical soil column with its lower end in water may be 
and frequently is at an appreciable distance above the water. From 
these data and as the result of tests by the writer and others, it can 
be said that a vertical soil column can take up by capillarity from a 
body of free water more water than it can. hold against gravity, if 
the free water be removed from the bottom of the soil column: that 
is, if the vertical tube is filled with soil and the lower end placed in 
a vessel of water and allowed to stand for a month or longer and the 
water is then removed from the tank, a part of the moistiu'e m the 
soil column will drain out. To repeat — a vertical soil column wiU 
take up by capillarity from a body of water more moisture than it 
can retain when the source of the w^ater is removed. In view of the 
above statements and the recorded experiments, it appears that capil- 
lary siphons maj^ occur in nature, as the result of physical lav\'s. 



CAPILLAEY MOVEMENT OF SOIL MOISTURE. 



63 



CAPILLARY MOVEMENT OF MOISTURE FROM A WET TO A DRY 

SOIL. 

■ As 1ms been stated previously, the movemeiit of moisture by capil- 
laritv' is much slower and not so extensive in the absence of free 
Avater as it is in the presence of free water. When a wet soil and a 
dry s^oil are in contact, gravity exerts an appreciable influence in the 
capillary movement of moisture. 

The experimental work so far done at Eiverside does not war- 
rant more than a few general statements. To give some idea of the 
nature of this work a few experimental results will be given. 

THE VERTICAL BOXES. 

The soil boxes were placed in vertical and horizontal positions onh^ 
In the vertical bo:ses the wet soil was placed in some cases on top, 
in otliers at the bottom, and in others the wet soil was placed in the 
middle section and dry soils at both ends. 

Nearly all boxes were 6 feet in length and the wet soil occupied 
one half this length and the dry soil the other half. 

JIOVESIENT or ilOISTUEE UPW^VRI). 

In table 43 arc given data of a few of the boxes in wdiich the soil 
moistened to the percentages shown were placed at the lower ends of 

Table 43. — Movement of moistvrc upicard in the, boxes. 



Days. 


Riverside soil, 
initial percentage. 


Idaho lava soil, 
•initial i)ercentage. 


Whittier soli, 
irjtial psrcentage. 


.20 per 
cent. 


10 per 
ceirt. 


14 per 
cent. 


20 per 

cent. 


25 per 
cent. 


40 per 
cent. 


30 per 
cent. 


1 
2 

4 

') 

G 

7 

8 

9 
11 
12 
U ■ 
16 
.23 
26 
37 
40 
49 
56 
Tl 
E6 


Inches. 
1.12 
2.25 
3.00 
8.37 
4.00 
4.50 
4. 82 
5.00 
5.3" 


Indies. Inches. 


Inches. 
1.50 


Inches. 


Inches. 


Inches. 





1.00 






1.25 


1.25 








2.25 
















1.83 








2.00 














1 






3.25 


4.50 




2.25 








2.75 








;7..-99 






S.SO 


1 . .. 




4.50 






8.87 -■ 


4.37 , 3.25 


7.75 






1.70 


1.00 
1.75 


10.75 


3.50 


6.83 




10.50 
11.00 




'"i2."75"' 


1 
















G.W 
















• 


14.25 






1 '" 











the boxes and air-dried soils at the upper ends. The table shows 
that the box containing the Eiverside' soil, with the lower half 20 per 
cent of moi.sture, the movement of the moisture up into the dry soil 
was about one-fourth as great in •! days as it was in 56 days. In the 
box of Riverside soil, containing 10 per cent moisture in the wet 



64 BULLETIIsT 835, IT. s. DEPARTMENT OF AGRICULTURE. 

pack, the movement of moisture into the dry soil the first 3 days was 
about one-fifth as great as in 71 days. 

The other data in the table show the rehitively rapid rate of mois- 
ture movement the first few days and the slowing- down of the rate 
of movement with the lapse of time. 

These results in connection with previous data for the flumes in- 
dicate that the larger part of capillary distril)ution of the water 
occurs while water is being applied and in the next two or three days 
thereafter. 

The last two columns of the table, which give data for the heavy 
Whittier soil, show the very slow and limited capilhuy movement of 
moisture in this class of soils. 

In the three boxes containing the Idaho lava-ash soil with rela- 
tively great capillary power, the movement of moisture up into the 
dry soil did not extend very far. In the box the wet pack of which 
contained 25 per cent of moisture the upward movement in 86 days 
was only 14.25 inches. The field capacity of this soil is from 20 to 
25 per cent or a little less than the percentage of moisture in the box 
just considered. 

In the box the wet pack of which contained 14 per cent of moisture 
the movement of the moisture upward was only 3| inches in 37 days. 

If the data in Table 43 were plotted as were the data for the 
flumes the resulting line would have a parabolic form. 

MO\'EMENT OF MOISTL'KE DOWNWARD. 

Table 44 is arranged to show the distance the moisture moved 
downward in the boxes after various periods of time, the moist soils 
being placed above the air-dried soils. The table shows about the 
same conditions as did the previous table, except that the rate and 
extent of movement of the moisture clowmward are considerably 
greater than with the wet soils below the cUy. The rate of move- 
ment downward is in proportion to the initial percentage of moisture 
contained in the wet soil. 

In the Eiverside soil containing 15 per cent of moisture, or about 
the field capacity, the extent of movement of the moisture at the end 
of the fourth day is approximately one-half the distance moved in 
36 da3'S. In the Idaho soil containing 20 per cent of moisture in the 
wet pack the moisture had moved in 36 days only about two and one- 
half times as far as it had at the end of 4 days. In the heavy 
Whittier soil the movement of the moisture even with a moisture con- 
tent in the wet pack ecpal to or greater than the field capacity is 
very slow and does not move to any great distance in 30 days. The 
data of this table, if plotted, as were the other data, would give a 
curve resembling a parabola. 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 
Table 44. — Movement of moisture downward from wet soil. 



65 



Days. 


Riverside soil, 
initial percentage. 


Idaho lava soil, Whittier soil, 
initial percentage. initial percentage. 


20 per 
cent. 


15 per 
cent. 


14 per 
cent. 


20 per 
cent. 


25 per 
cent. 


41 per 
cent. 


30 per 
cent. 


1 
2 
3 
4 
5 
7 
8 
9 
13 
16 
22 
27 
31 
36 
41 
43 
49 
71 
76 


Inch cs. 
4.50 


Inches. 
4.00 
5.75 
6.37 
7.00 


Inches. 
0.75 


Inches. 
2.00 


Inches. 
3.00 
5.25 


Inches. 


Inches. 






7.50 












4.75 
5.00 










1.75 












9.00 






11.75 


8. .50 










4.00 


8.66 








14. 00 
15. 75 
17.25 


9.50 
10.75 
11.25 
12.00 


12.00 






4.50 


8.50 






15.25 














21.50 






16.25 
17.25 


2.25 


2.00 


14.25 
15.00 




11.50 










25.50 












16.25 




12.75 












21.50 
22.25 































COMPARISON OF CAPILLARY SrOVEMENT OF MOISTURE UPWARD AND DOWNWARD FROM 

A BODY OF WET SOIL. 

A series of experiments were outlined to determine the relative 
extent and rate of movement of moisture upward and downward 
from a body of soil containing a known percentage of moisture. In 
this experiment a section in the middle of the box w^as filled with 
wet soil and air-dried soil was packed at both ends. The box was 
then placed vertically. In this experiment the capillary movement 
occurred with gravity downward and in opposition to gravity. There 
was a secondary factor which must be considered, and that is the 
gradual concentration of moisture in a wet soil at the lower end of a 
vertical column due to gravity. That is, while the middle part of the) 
flume was filled with a .soil containing a uniform percentage of 
moisture it would be found after a few days, depending upon the de- 
gree of wetness of the soil, that there was a greater percentage of 
moisture near the bottom than near the top of the wet soil column. 
The more nearly the soil was wetted to the point of capillary satura- 
tion the greater would be the difference in percentage of moisture 
near the bottom and near the top. 

Table 45 shows the upward and downward movement of moisture 
in two of the boxes. 

The box containing the Idaho soil was 8 feet long and the middle 
32 inches was packed with wet soil. There was an equal length of air- 
dry soil at each end. 

The box containing the Eiverside soil was 8 feet long and the mid- 
dle 4 feet was packed with wet soil. 
147697°— 20— Bull. 835 5 



66 



BULLETIKT 835, U. S. DEPARTMENT OF AGRICULTURE. 



Table 45. 



-Movement of moisture upward mid doumivard, from soils contain- 
ing an initial moisture content of 15 per cent. 



Time 
in 

days. 


Idaho soil. 


Riverside soil. 


Distanc 


s moved. 


Tlelation 

of lip 
to down. 


Distance moved. 


Relation 

of up 
to down. 


Up. 


Down. 


Up. 


Down. 


2 

4 

5 

6 

10 

13 

17 

23 

31 

36 

43 

52 

71 

76 


Inches. 
1.50 
1.50 


Inches. 


Per cent. 


Inches. 
2.25 


Inches. 
3.50 


Per cent. 
64 


2.25 


67 


2.62 
2.88 
3.75 


7.25 
7.75 
10.00 


36 
37 
37 


2.30 


3.20 


73 


3.00 


4.50 


67 


5.75 


13.00 


44 


4.12 
4.50 
4.80 


5.50 
6.75 
7.00 


75 
67 
68 








6.50 
6.75 


18.25 
19.00 


35 
36 


5.37 
6.00 
6.25. 


8.37 
9.12 
9.24 


64 
66 
67 





















Table 45 sliows by iDercentage the relation of the upward 
movement of the moisture to the downward movement. After the 
first day or two the relation of the upward movement to the down- 
ward movement remains rather constant. The table shows the rela- 
tive rapid rate of movement of moisture the first few days and the 
slower rate with the lapse of time. If the data in Tables 41 and 42 
showing the upward and downward movement of moisture in sepa- 
rate flumes are compared, the same relative relation is found as 
found in Table 45. 

The above data indicate the part gravity plays in soil-moisture dis- 
tribution. Generally speaking, the lighter the soil the less is the up- 
ward movement of the moisture as comjoared with the downward 
movement. It also appears that the greater the percentage of mois- 
ture tlie greater the downward movement as compared with the up- 
ward. 

The limited data above presented, when considered with many 
others in the original records, would lead to the conclusion that 
under irrigation much moisture may be carried below the root zone 
of plants, and that moisture once carried below the root zone of 
plants will probably not be again brought within the root zone in 
sufficient quantity to be of material benefit to the crop of that season, 
and hence will be lost to the plant. 

THE MOVEMENT OF MOISTURE FROM WET TO DRY SOIL IN HORIZONTAL BOXES. 

The capillary movement of soil moisture in a horizontal direction 
as found in the horizontal boxes is greater in extent than the upward 
movement in the vertical boxes, but not so great as the downward 
movement. There are g-iven in Table 46 the results of three tests 



CAPILLARY MOVEMENT OF SOIL MOISTUEE. 



67 



T.VBLE 46. — Horizontal movement 
of soil moisture in Riverside 
soil. 



Time 

in 
days. 


Initial moisture. 


10 per 

cent. 


taper 
cent. 


20 per 

cent. 


1 
2 
3 

4 
5 
7 

10 
12 
Ifi 
19 
21 
24 
29 
40 
4H 
49 
51 
54 


Indies. 
0.75 
1.25 
1.50 


Iftches. 
4.00 
5.50 
6.25 


Indies. 
5. 75 
7.00 
8.25 
9.25 
9.75 
10.75 


1.83 


7.50 


3.00 


9.50 


13.50 
15.00 


5.00 
5.25 


11.00 




16.25 




13.25 




17.75 
19.50 
23.25 


5.50 


18.25 




19. 00 




23. 50 




'19. 25 







with the Riverside soil, with 10. 15, and 20 per cent moisture in the 
wet soil. The table shows, like the preceding ones, that the rate and 
extent of movement of the moisture varies as the initial percentage 
of moisture in the wet pack. There 
is also sliown the rapid moisture 
movement for the first few days and 
a slowing down of this rate with 
lapse of time. These data if plotted 
would also give a curve of a para- 
bolic form. It is surprising to find 
so great an extent of movement 
of moisture in a horizontal direc- 
tion when compared with the down- 
ward movements as shown in Table 
42. If the difference in movement 
of moisture in the several boxes 
as representing the upward, down- 
ward, and horizontal can be attrib- 
uted onl}^ to gravity, and this ap- 
pears to be true, then gravity is a 
most important factor in the capil- 
lary distribution of soil moisture. 

M'liile the experiments above noted are not sufficient in number 
to warrant any final conclusion, in connection with many others 
not contained in this report they indicate the probably distribution 
of moisture. 

These data are in accord Avith results obtained by others (T), (9), 
(10), (18). 

DISTRIBUTION OF MOISTURE IN BOXES CONTAINING WET AND DRY SOIL. 

It is interesting to observe the distribution of the moisture tlirough- 
out the entire length of the soil in the boxes at the termination of the 
experiments. It is interesting to observe the movement of moisture 
in .quantity from the wet soil into the air-dried soil, and in the verti- 
cal boxes to note the relative percentages of moisture moved upward 
and downward. Table 47 gives the distribution of moisture at the 
end of the experiment in the soil boxes just previously discussed. 

In Table 47 are given the kind of soil and the initial percentage 
of moisture contained in the wet soil as placed in the boxes at the be- 
ginning of the experiment. The percentages of moisture and the 
distances inclosed between the heavy lines in the body of the table 
show the original wet area of soil in the boxes and the remaining 
figures outside of the heavy lines show that part of the original air- 
dried soil with the corresponding percentages of moisture found at 
the end of the experiment. For instance, in the first two cohunns the 
first two lines indicated by minus 5 inches and minus 2 inches repre- 



68 



BULLETIN 835, U. S. DEPARTMENT OF AGRICULTURE. 



sent that part of the soil cokimn immediately below the original wet 
soil area. Likewise the distances 34 inches and 40 inches at the bot- 
tom of the table represent that part of the original air-dried soil on 
top of the original wet soil. The other part of the table has a similar 
arrangement, except that the distances Avere taken from the bottom of 
the boxes. Referring to the Riverside soil it is found that the distri- 
bution of moisture from the bottom of the box upward is quite uni- 
form until near the upper extremity of the original wet area. At a 
distance of 47 inches 9.19 per cent of moisture is found, while at 50 
inches there is 6.6 per cent of moisture. In corresponding distances 
at the bottom of the box, represented by 22 inches and 18 inches, re- 
spectively, a much less variation in the percentage of moisture is 
found. 

Table 47. — Dlstrlhution of mo'n^iure hij poTentaere in the soil boxes. 



Idaho soil, initial 
moisture 20 per cent. 


Eiversidc soil, initial moisture 
15 per cent. 


Distance. 


Moisture 
content. 


Distance. 


Moisture 
content. 


Distance. 


Moisture 
content. 


Inches. 
-5 
_2 


Per cent. 
9.46 
11.31 


Inches. 
3 
6 
9 
12 
15 
18 


Per cent. 
. 5. 05 
7.08 
8.25 
8.61 
9.09 
9.42 


Inches. 
5 
8 
11 
15 
19 
22 


Per cent. 
4.74 
6.90 
8.05 
8.81 
9.04 
S. 75 


12 
IS 
24 
28 
31 


14.09 
14.46 
15. 05 
16.00 
15.44 
15.51 
15.40 


22 
25 
2ii 
31 
34 
37 
40 
44 
47 


10. 79 
10.20 
10. 34 
10. 60 
10. 00 
9.86 
9.38 
9.50 
9.19 


24.5 
27 
30 
34 
38 
42 
46 
50 
54 
58 
<i2 
66 
70 


11.40 

12. 37 

11.28 

11.50 

11.05 

11.13 

10.45 

10.40 

9.35 

9.48 

9.43 

8.92 

8.28 


34 

40 

























50 

58 


6. 60 
3.90 




















! 




74 
77 


4.99 
3.28 




1 






1 





It would seem from Table 47 that gravity has played its part in 
conjunction with capillarity in a rather uniform distribution of soil 
moisture from the wet .soil area to the dry soil area. Upon the other 
hand it is found in taking the moisture percentages that gravity has 
very materially retarded the upward movement of the soil moisture. 
It is found, for in.stance, that the percentage of moisture found im- 
mediately below the original wet soil area is almost double the per- 
centage of moisture found immediately above the upper end of the 
original wet soil area. 

If such a condition as this maintains in the field, and there is no 
reason to believe it does not, then we can expect that capillarity and 
gravity will tend to a deep penetration of the moisture. The figures 



CAPILLARY MOVEMENT OF SOIL MOISTURE. 69 

in Table 47 and those immediately preceding show conclusively that 
capillarity and gravity tend to move the soil moisture downward to 
considerable depths and in about twice the quantity that the moisture 
inoves upward. Add to this factor copious irrigation and it is read- 
ily seen how even capillarity can assist and does assist in the waste 
of irrigation water by deep penetration. 

In none of the data presented just above has the original ^vet soil 
contained a percentage of moisture differing nuich from that which 
VN-ould be found in the field immediately following an irrigation. 

Sufficient tests have not been made to warrant a final conclusion as 
to the ultimate importance of the deep penetration of moisture, by 
capillarity, in conjunction with gravity. 



REFERENCES. 

(1) Alwat, F. J., and Clark, Y. J. 

1912. A Study of the Movement of Water in a Uniform Soil under Agri- 
cultural Conditions. In Neb. Exp. Station, 25tli Annual Report, 
p. 247-287. 

(2) Banyancos, C. J. 

1915. Effect of Temperature on Movement of Water Vapor and Capillary- 
Moisture in Soils. Jour. Agri. Research, U. S. No. 4, p. 141. 
(.3) Briggs, L. J., and Lapham, M. H. 

1902. Capillary Studies and Filtration of Clay from Soil Solutions, U. S. 
Dept. Agri., Bur. Soils Bui. 19, p. 14-24. 
(4) Bkiggs, L. J., and McLean, J. W. 

1907. The Moisture Equivalent of Soils. U. S. Dept. Agri., Bur. Soils 
Bui. 4.5. 
(.5) Briggs, L. J. 

1897. The Mechanics of Soil JMoisture. U. S. Dept. Agr., Bur. Soils Bui. 
10, p. 6. 

(6) Buckingham, Edgar. 

1907. Studies on the Movement of Soil Moisture. TJ. S. Dept. Agr., Bur. 
Soils Bui. 38, p. 14. 

(7) Burr. W. W. 

The Storage and Use of Soil IMoisture. Neb. Exp. Sta. Researcli Bui. 5. 

(8) Free, E. E. 

riant World, vol. 14, No. 2, p. 164. 

(9) Harris, P. S. 

1917. Soil Moisture Studies under Irrigation. Utah Exp. Station Bui. 159. 

(10) 1917. Movement and Distribution of Moisture in the Soil. Jour. Agr. Re- 

search, vol. 10, No. 3. 

(11) King, F. H. 

1899. Wis. Exp. Sta. 16th An. Report, p. 214. 

(12) 1888. Soil Physics, Wis. Exp. Sta., 6th An. Report, p. 189. 

(13) LOUGHRlDGE, R. H. 

1894. The Capillary Rise of Water in Soils. Cal. Exp. Station, Report 
1892-1894, p. 91. 

(14) RisTER and Weby. 

1904. Irrigation et Drainage, p. 66, Paris. 
(1.5) Smith, Albert. 

1917. Relation of the Mechanical Analysis of the Moisture Equivalent 
of Soil. Soil Science, vol. 14, No. 6, p. 471. 

(16) Stein METZ, Charles P. 

1917. Engineering Mathematics, New York. 

(17) WiDTSOE, J. A. 

1914. Principles of Irrigation Practice, p. 17. New York. 

(18) Widtsoe, J. A. and McLaughlin, W. W. 

1902. Irrigation Investigations in 1901. Utah Exp. Sta. Bui. 80. 

(19) 1912. The Movement of Water in Irrigated Soils. Utah Exp. Sta. Bui. 

115. 
70 



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